[Travis] Do not install a custom libc++ and libc++abi

I must have done that because libstdc++ was not sufficiently close to
modern standards at the time, but the right thing to do is to rely on
the Standard Library provided by the system and/or the compiler.
Otherwise we're always testing against the same standard library,
which is not realistic.

.travis.yml

@@ -67,29 +67,6 @@ install:
     fi
   - cmake --version
 
-  ############################################################################
-  # Install libc++ and libc++abi
-  ############################################################################
-  - |
-    if [[ "${LLVM_VERSION}" != "" ]]; then
-      LLVM_DIR=${DEPS_DIR}/llvm
-      LLVM_URL="http://llvm.org/releases/${LLVM_VERSION}/llvm-${LLVM_VERSION}.src.tar.xz"
-      LIBCXX_URL="http://llvm.org/releases/${LLVM_VERSION}/libcxx-${LLVM_VERSION}.src.tar.xz"
-      LIBCXXABI_URL="http://llvm.org/releases/${LLVM_VERSION}/libcxxabi-${LLVM_VERSION}.src.tar.xz"
-
-      mkdir -p ${LLVM_DIR} ${LLVM_DIR}/build ${LLVM_DIR}/projects/libcxx ${LLVM_DIR}/projects/libcxxabi
-      travis_retry wget -O - ${LLVM_URL}      | tar --strip-components=1 -xJ -C ${LLVM_DIR}
-      travis_retry wget -O - ${LIBCXX_URL}    | tar --strip-components=1 -xJ -C ${LLVM_DIR}/projects/libcxx
-      travis_retry wget -O - ${LIBCXXABI_URL} | tar --strip-components=1 -xJ -C ${LLVM_DIR}/projects/libcxxabi
-      (cd ${LLVM_DIR}/build && cmake .. -DCMAKE_INSTALL_PREFIX=${LLVM_DIR}/install)
-      (cd ${LLVM_DIR}/build/projects/libcxx && make install -j2)
-      (cd ${LLVM_DIR}/build/projects/libcxxabi && make install -j2)
-
-      export CXXFLAGS="-isystem ${LLVM_DIR}/install/include/c++/v1"
-      export LDFLAGS="-L ${LLVM_DIR}/install/lib -l c++ -l c++abi"
-      export LD_LIBRARY_PATH="${LD_LIBRARY_PATH}:${LLVM_DIR}/install/lib"
-    fi
-
 
 before_script:
   - cd ${TRAVIS_BUILD_DIR}