Revert "Add fallback to bswap_16 for gcc on Windows (closes #377)"

This reverts commit 1d3c6401f8186b54453ba64606758a90b6ec3fca.

TravisCI reports:
../src/.libs/libmodbus.so: undefined reference to `__builtin_bswap16'

with gcc 4.6.3

src/modbus-data.c

@@ -40,8 +40,6 @@
 #  define GCC_VERSION (__GNUC__ * 100 + __GNUC_MINOR__ * 10)
 #  if GCC_VERSION >= 430
 // Since GCC >= 4.30, GCC provides __builtin_bswapXX() alternatives so we switch to them
-#    undef bswap_16
-#    define bswap_16 __builtin_bswap16
 #    undef bswap_32
 #    define bswap_32 __builtin_bswap32
 #  endif