Remove unused ps_lgetLDT etc. on Solaris/x86 [PR25981]

Remove unused ps_lgetLDT etc. on Solaris/x86 [PR25981]

As reported in PR build/25981, a future Solaris 11.4 update will soon
remove the short i386 register names like SS etc. from <sys/regset.h>.
They could leak into user code (e.g. via <signal.h> -> <sys/signal.h> ->
<sys/ucontext.h>) and pollute the user namespace.  Affected code would
have a hard time avoiding the issue: LLVM is one of those.

While the short names are required to be present by the i386 psABI, that
document only demands that they exist in <ucontext.h>, which is what the
upcoming update assures.

With this change, in a 64-bit-default configuration, procfs.c fails to
compile on Solaris/x86:

/vol/src/gnu/gdb/hg/master/git/gdb/procfs.c: In function 'ssd* procfs_find_LDT_entry(ptid_t)':
/vol/src/gnu/gdb/hg/master/git/gdb/procfs.c:1643:18: error: 'GS' was not declared in this scope
 1643 |   key = (*gregs)[GS] & 0xffff;
      |                  ^~
make[2]: *** [Makefile:1607: procfs.o] Error 1

Initially I meant to provide a definition using the planned replacement
macro, but closer inspection revealed a better way.  procfs_find_LDT_entry
and its helper proc_get_LDT_entry are only used to implement ps_lgetLDT,
one of the callback functions required by libthread_db.so.1
(cf. <proc_service.h>).  While that function is still documented as being
required even in Solaris 11.4, I found that calls to it had been removed
long ago in Solaris 9, so just removing the three functions above is the
easiest fix.

The following patch does just that.  It compiled successfully on
amd64-pc-solaris2.11, however, as reported in PR gdb/25939, master is
completely broken on Solaris since the multi-target patch.  The patch
applies cleanly to the gdb-9 branch and there I could test it
successfully.

	PR build/25981
	* procfs.c [(__i386__ || __x86_64__) && sun] (proc_get_LDT_entry,
	procfs_find_LDT_entry): Remove.
	* procfs.h [(__i386__ || __x86_64__) && sun] (struct ssd,
	procfs_find_LDT_entry): Remove.
	* sol-thread.c [(__i386__ || __x86_64__) && sun] (ps_lgetLDT):
	Remove.