Changes

gsmtap_util: Get rid of unused wq

struct gsmtap_inst went private in
1584b2ac39292aedddb8abd165957912fe9399c0 and
f38077ee6ab429ef3f0881a8ca7de4877032358a, which were merged prior to
libosmocore release 1.10.0. That release bumped the LIBVERSION major
counter, and hence that struct is officially not available and protected
by LIBVERSION ABI since then.

That means we can safely remove it from the implementation since nobody
compiled against this libversion should be using those fields directly anymore.

Related: OS#6213
Change-Id: I4f1cf168c230471a6d7be4bb065d7105a993349f