Meson: Remove default_library=both from default_options

It is inconsistent with what all other Meson projects does.

It also prevents static linking to freetype when it is a subproject. That's because default_options in a subproject takes precedence on values set on the main project. For example building GStreamer with -Ddefault_library=static would still dynamic link on freetype, unless user also set -Dfreetype2:default_library=static.