MuonTrackQuerry - Remove constexpr to avoid clang-10 build failure

Hi, apologies my last MR introduced a massive failure in the clang-10 builds as constexpr and the call of std math functions do not like each other: https://bigpanda.cern.ch/compsview/?nightly=master_Athena_x86_64-centos7-clang10-opt&rel=2021-05-10T2130&ar=x86_64-centos7-clang10-opt&proj=Athena

I removed the constexpr to fix the error. Tagging: @ssnyder, @christos, @wlampl