gcm.c: remove dead store
gcm.c: remove dead store

Found with Clang's `scan-build` tool.

The value written to `hi` is never used, resulting in a warning. Remove
the dead store to get rid of the warning.

NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls