linux(4): Retire unneeded initialization

Both uc_flags and uc_link are zeroed above. On amd64 and i386 the
uc_link field is not used at all. The UC_FP_XSTATE bit should be set
in the uc_flags if OS xsave knob is turned on (and xsave is implemented).

MFC after: 2 weeks

(cherry picked from commit 0b5d5dc3767fcf2f0198f0f9549cb23983e0ee3e)