gdbsupport: remove gdb::optional

The previous patch migrated all the uses of gdb::optional to use
std::optional instead,  so gdb::optional can be removed entirely
as well as the self-tests which came with it.

Change-Id: I96ecd67b850b01be10ef00eb85a78ac647d5adc7
Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Pedro Alves <pedro@palves.net>