ruby: remove obsolete build dependences

ruby: remove obsolete build dependences

Ruby 3.3.0 no longer bundles the readline module[1] so readline is no
longer a build dependency[2].

Ruby 3.2.0 no longer bundles the gdbm module[3] so gdbm is no longer a
build dependency.

[1] https://github.com/ruby/ruby/commit/59fd67fc3d405e529e038172e769ff20a8fb5535
[2] https://github.com/ruby/ruby/commit/172077232e7f40b4dfedf6a5434c252b3d326fab

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>