Remove unnecessary const qualifier from cast (#4666)

Const does not make sense here, and compiler actually throws warning
(error with -Werror) when you would try to compile it.

    In file included from include/flatbuffers/flexbuffers.h:24,
                     from include/flatbuffers/idl.h:26,
                     from include/flatbuffers/code_generators.h:22,
                     from src/code_generators.cpp:17:
    include/flatbuffers/util.h: In function ‘int flatbuffers::FromUTF8(const char**)’:
    include/flatbuffers/util.h:325:44: error: type qualifiers ignored on cast result type [-Werror=ignored-qualifiers]
       if ((static_cast<const unsigned char>(tmp) << len) & 0x80) return -1;  // Bit after leading 1's must be 0.
                                            ^
    cc1plus: all warnings being treated as errors

This warning caught by gcc8:

    $ g++ --version
    g++ (GCC) 8.0.1 20180222 (Red Hat 8.0.1-0.16)