xtensa: drop unused function fast_coprocessor_double

fast_coprocessor_double is not used since commit c658eac6 ("[XTENSA]
Add support for configurable registers and coprocessors"). Remove it.
There should be no coprocessor exceptions generated in the exception
handling paths while PS.EXCM is set.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>