chore: remove -Werror

As much as it disappoints me to allow to allow warnings to be passed in
CI. Currently warnings are emitted when compiling the tests (Catch2
upgrade) and also from legacy sundown package (external-ish dependency).

Given the product maturity and lack of maintenance will allow warnings.

.circleci/config.yml

@@ -4,7 +4,7 @@ aliases:
   - &test-base-legacy
     working_directory: /tmp/drafter
     environment:
-      CXXFLAGS: '-Werror -Wno-error=unused-function'
+      CXXFLAGS: '-Wno-error=unused-function'
     steps:
       - checkout
       - run: git submodule update --init --recursive