Drop the check for the fips_enabled symbol

The buildcheck.py script now check the build kernels against a reference list of symbols exported to modules to catch ABI breackages.

Unfortunately, it caused build failures since we don't export the fips_enabled symbol. Its presence depends on the CRYPTO_FIPS config option, which depends on the MODULE_SIG option: since we don't sign our kernels (yet), the CRYPTO_FIPS option is automatically disabled by Kconfig when make oldconfig is run during the build process.