builtins.h: remove fake __bic_SR_register_on_exit definition

This definition is wrong, because the offset on the stack is not
a constant. See this link for some ideas for how to actually do it:
rust-embedded/wg#20 (comment)

Author: alexeicolin

src/include/libmspbuiltins/builtins.h

@@ -14,16 +14,9 @@
       : : [count] "i" ((n) / 3 - 3) \
     )
 
-#define __bic_SR_register_on_exit(bits) \
-    __asm__ volatile ( \
-        "bic     %[b],    24(r1)\n" \
-        : : [b] "i" (bits) \
-    )
-
 // These are fake "built-ins" for Clang: implemented in C in a trivial way, and
 // compiled with gcc. We do this because it is easier than implementing actual
 // builtins Clang calls that come from compiler-rt (few are implemented there).
 uint32_t mspbuiltins_mult32(uint32_t a, uint32_t b);
 uint16_t mspbuiltins_div16(uint16_t a, uint16_t b);
-
 #endif // _MSP_BUILTINS_H