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>

arch/xtensa/kernel/coprocessor.S

@@ -145,13 +145,6 @@ ENDPROC(coprocessor_flush)
  *	     <  VALID_DOUBLE_EXCEPTION_ADDRESS: regular exception
  */
 
-ENTRY(fast_coprocessor_double)
-
-	wsr	a0, excsave1
-	call0	unrecoverable_exception
-
-ENDPROC(fast_coprocessor_double)
-
 ENTRY(fast_coprocessor)
 
 	/* Save remaining registers a1-a3 and SAR */