Remove unneeded :expected_type overrides

This use of an :expected_type check is no longer required so these
mutations do not need to override the base class implementation of
`find_object`.

The `:expected_type` checks date back to when ids were all represented
by a single generic `ID` type.  This has been replaced with specific
types of the form `GlobalIdType[::Project]`, `GlobalIdType[::Issue]`,
etc.

These specific types handle the type checking when the GID object is
parsed.  When an argument is defined as `GlobalIdType[::Project]`
passing a GID like `gid://gitlab/Issue/1` will fail before the
`:expected_type` check is ever performed.

See https://gitlab.com/gitlab-org/gitlab/-/issues/257883 for more
context.