Remove some IRIX ifdeffery

[vincele]

Intentionally not touching the other ones:

• engine/src/endianness.h
• engine/src/lin_time.cpp
• engine/src/posh.h

As they are more involved, this is a first step in the right direction (hopefully)

Fixes: issue #995 (partly)

Code Changes:

• Have the PR Validation Tests been run? compiles & run on linux/musl
• This is a documentation change only

Issues:

• Still some work to do after this first step

Purpose:

• What is this pull request trying to do? Remove dead code
• What release is this for? Next
• Is there a project or milestone we should apply this to? Next

[BenjamenMeyer]

This should be pretty good to go. We don't support IRIX... leaves me to wonder who used it in the past; but we don't now and even if we did we want to minimize this kind of thing too.

[stephengtuggy]

Looks good so far. Just to double-check though, I would like to have someone test run the game on each of our three major supported platforms with these changes. (Windows, Linux, and macOS.)

[royfalk]

I went looking for glEnable( GL_SHARED_TEXTURE_PALETTE_EXT ) irix
and found another effort to revive the game from 10 years ago.

https://sourceforge.net/p/vegastrikevo/blog/2015/06/old-vegastrike-framework-inspection/

I glanced through and there may be some art and assets worth looking at.

[royfalk]

LGTM.

The glEnable( GL_SHARED_TEXTURE_PALETTE_EXT ); seems to also apply to other platforms. Something to keep in mind maybe?