various: Remove HAVE_OBSCONFIG_H ifdefs

[gxalpha]

Description

Removes HAVE_OBSCONFIG_H ifdef-checks and marks the definition for removal in 32.0 (as removing it could be considered a breaking change, in case there are any third-party plugins checking for it).

Motivation and Context

With the removal of all legacy code paths, obsconfig.h always exists and the HAVE_OBSCONFIG_H compile definition always gets set. As such, it's no longer necessary to check for it.

How Has This Been Tested?

Tested with CI that all builds still compile.
Tested with git grep that the definition line in cmake is now the only instance remaining of HAVE_OBSCONFIG_H.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[norihiro]

I'm sorry that there is no module name various which is written in the commit message.