GHA/macos: delete misplaced CFLAGS, drop redundant CMake option

With macOS there is a long-term struggle with deprecation warnings.
In curl they occur with LDAP, SecureTransport and in docs/examples.

There are three ways to fix them:
- by CFLAGS `-Wno-deprecated-declarations` as a workaround.
- by CFLAGS `-mmacosx-version-min` set to a version where the the
 feature was not deprecated.
- by CMake option `-DCMAKE_OSX_DEPLOYMENT_TARGET=`.

In GHA CMake jobs, all three were used, and `-mmacosx-version-min` was
set in a bogus way. Delete that bogus option, and delete the lone,
redundant CMake option too.

In a future commit I might replace the suppression option to properly
setting the target OS.

Follow-up to dfdd978f7c #13491
Cherry-picked from #14097

.github/workflows/macos.yml

@@ -210,8 +210,7 @@ jobs:
     runs-on: 'macos-latest'
     env:
       CC: ${{ matrix.compiler.CC }}
-      # '-Wno-deprecated-declarations' required for LDAP and BUILD_EXAMPLES
-      CFLAGS: '-DCMAKE_C_FLAGS=-mmacosx-version-min=10.15 -Wno-deprecated-declarations'
+      CFLAGS: '-Wno-deprecated-declarations'  # Required for LDAP and BUILD_EXAMPLES
     strategy:
       fail-fast: false
       matrix:
@@ -221,7 +220,7 @@ jobs:
         build:
           - name: OpenSSL
             install: nghttp2 openssl gsasl
-            generate: -DOPENSSL_ROOT_DIR=$(brew --prefix)/opt/openssl -DCMAKE_OSX_DEPLOYMENT_TARGET=10.9 -DCURL_USE_GSASL=ON
+            generate: -DOPENSSL_ROOT_DIR=$(brew --prefix)/opt/openssl -DCURL_USE_GSASL=ON
           - name: LibreSSL
             install: nghttp2 libressl
             generate: -DOPENSSL_ROOT_DIR=$(brew --prefix)/opt/libressl -DCURL_DISABLE_LDAP=ON -DCURL_DISABLE_LDAPS=ON -DBUILD_EXAMPLES=ON