Remove #if __GNUC__ conditions and its cases

[mloskot]

As pointed in #382 (comment), since we are at C++14 already, cases like this one can go

nanodbc/nanodbc/nanodbc.cpp

Lines 42 to 43 in 5b52ff0

	#elif defined(__GNUC__) && (__GNUC__ < 5)
	#include <cwchar>

The oldest GCC version we are testing is

nanodbc/.github/workflows/ci-linux.yml

Line 45 in 54a3f2e

- { cxxstd: '14', cxx: g++-5, cc: gcc-5, os: ubuntu-18.04 }