Remove obsolete Tru64 UNIX V5.1B support

[Arnaud Charlet]

The gnattools and gcc/ada parts are OK, except for the comment removal in
s-tassta.adb: this comment is still useful, and needs to be revisited at
some point ratheer than removed silently as you did, to understand
why we can't use a when E: others => construct.

So either remove the s-tassta.adb hunk, or extend the comment, but removing
it would be wrong.

Arno