gcore: Remove unused typedefs.

These are no longer needed after commit 4965ac059da1 which used
PT_GETREGSET to fetch NT_PRSTATUS and NT_FPREGSET.

Reviewed by: markj, emaste
MFC after: 2 weeks
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D35665

(cherry picked from commit d2a3c30a511d4802412a102e9fa44f810605a872)