gdb_server: Remove leftover testing and x86 parts

gdb_server was removed in commit 0f669132 ("kernel: remove
gdb_server"), but still has a testcase that sets CONFIG_GDB_SERVER=y,
and some code in arch/x86/debug.

Remove the leftover parts. This also gets gets rid of undefined
references to the CONFIG_GDB_SERVER symbol.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>