sfxge(4): cleanup: remove unused EFX preempt macros

The EFSYS_PREEMPT_DISABLE() and EFSYS_PREEMPT_ENABLE() macros
were used to ensure correct timing of I2C operations. The APIs
for I2C operations have been removed, so these macros have no
callers.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week