Drop unneeded C++ compiler flag additions to AM_CXXFLAGS. Re #92

The AX_CXX_COMPILE_STDCXX_11 macro takes care of doing so if needed
already.
9 jobs for master in 10 minutes and 42 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Prepare
passed #232106374
prepare

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  Compile
passed #232106381
compile:alpine-3.9-musl

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passed #232106376
compile:debian-8-full

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passed #232106375
compile:debian-8-mini

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passed #232106378
compile:debian-9-full

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passed #232106377
compile:debian-9-mini

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passed #232106379
compile:fedora-29-clang

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passed #232106380
compile:fedora-30-clang

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  Archive
passed #232106382
archive

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