loader.efi: remove extra extern ST

The definition for 'ST' is in efilib.h, so we don't need extern ST here.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D32225

(cherry picked from commit 8ea95b2fbab8eb891c4191c1879199685951b1f6)