gdbsupport: Replace gdb::invoke_result with std::invoke_result

Given that GDB now requires C++17, we can replace gdb::invoke_result
with std::invoke_result which is provided by <type_traits>.

This patch also removes gdbsupport/invoke-result.h as it is not used
anymore.

Change-Id: I7e567356d38d6b3d85d8797d61cfc83f6f933f22
Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Pedro Alves <pedro@palves.net>