[gdb/testsuite, tclint] Drop lreverse

[gdb/testsuite, tclint] Drop lreverse

When running tclint with lib/future.exp, I get:
...
$ tclint lib/future.exp
$exp:756:5: redefinition of built-in command 'lreverse' [redefined-builtin]
...

The code was added to handle pre-7.5 tcl versions without lreverse.

Since we now require Tcl 8.6.2 (as per PR testsuite/33205), drop this.

Tested by rerunning tclint.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
PR testsuite/33403
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33403