[meson] Remove `default_library=both` from `default_options`.

It is inconsistent with what all other Meson projects do.

It also prevents static linking to FreeType when it is a subproject 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 dynamically link on FreeType unless the user also sets

* meson.build: Updated.

* .gitlab-ci.yml: Retain `both` option for testing.
2 files changed