Remove gcc 2_95 workaround.

[gogagum]

C++11 is now a minimal required C++ version for boost::format. This means that old gcc version (older then gcc 4.x) support is no longer required. This PR removes unused workaround for std::char_traits and std::allocator, which was made for gcc 2.95.

[jeking3]

Why didn't this run all the CI tests? :|

[gogagum]

@jeking3 I turned on CI runs on my fork initially, so github decided not to run tests again, when the pull request was made. So the lack of CI runs is not caused by changes. Now I turned off tests in my fork and reran CI. appveyor CI succeded. Github CI now has a requirement of approval from a maintainer.