gitk: _search_exe is no longer needed

The _search_exe variable allows specifying the suffix used for executables,
typically {} on unix, .exe on Windows. But, the override code is now
used only on Windows, so _search_exe is no longer needed. Eliminate it.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>