Hardware watchdog support was removed from heart in R13A, but there were
still some vestiges in the code and the documentation.

- Remove mentions of the HW_WD_DISABLE variable, as it's no longer used.
- Remove the HEART_BEAT_BOOT_DELAY variable, as it was only used for the
  hardware watchdog.

release_handler_SUITE:otp_10463_upgrade_script_regexp reads
kernel.appup and tries to fetch upgrade instructions for upgrade from
current version to current version. The point of the test is to see
that release_handler can read an appup with regexps.

In some cases, however, kernel.appup does not support upgrade from
current to current version, since this is not necessary in real
life. Therefore this test is now updated to use a dummy application
(release_handler_SUITE_data/regexp_appup/app1) for this test.

The upgrade instruction 'remove_module' was added in OTP R7B (and
possibly in a patch in R5B or R6B, ticket OTP-3477), and translates to
the low level instruction 'remove', but adds the parameter DepMods
(modules on which Mod is dependent). The ticket says that
"remove_module should be added for symmetry with the add_module
instruction". remove_module was never documented or tested, and it was
never mentioned in a release note. It therefore seems to be low risk
in removing it.

The correct instruction to use when removing a module is

{delete_module,Mod}

which was added in OTP R10B and which is also documented and
tested. This translates to low level instructions 'remove' and 'purge'
i.e. the module is brutally purged after setting the current code to
old. This hardcoded brutal purge is the reason why PrePurge and
PostPurge parameters can not be given with the delete_module
instruction.

The parameter DepMods which was inclued in the remove_module
instruction does not exist for delete_module. From the documentation's
point of view, this is the same for add_module, and thus the two
instructions {add_module,Mod} and {delete_module,Mod} are now
symmetric. However, in the code there is a second instruction for
adding a module, {add_module,Mod,DepMods}, which is not documented. To
add symmetry even for this, {delete_module,Mod,DepMods} is now also
added. Documentation is added for all instructions.

OTP-11529

It was possible to insert {Mod::atom(),Vsn::term()} instead of
Mod::atom() in the modules list in a .app file. This was not visible
in the documentation of .app files, but it was visible in the
documentation of application:load/[1,2] (where the .app file term can
be used directly as first argument).

The Vsn part was never used, so this possibility has now been removed.

The test is added in release_handler_SUITE since this makes it
possible to test the full upgrade instead of just a simple library
test of supervisor:code_change.

Changes to the mechanism for upgrading the emulator in OTP R15 was
erronously not handled in release_handler:upgrade_app, downgrade_app,
upgrade_script and downgrade_script. This has been corrected,
including test and documentation.