x86: remove unused T_USER flag

It stopped being used in 3c256f5395aa, when trap() was reorganized to
have separate switch statements for user and kernel traps. Remove the
two leftover references and the flag itself.

Reviewed by: kib
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D33253

(cherry picked from commit 03b3d7bbec99742910e2d0d5e7c23754eca9b2c4)