ci: Stop caching submodules, no consistent way to detect changes

There doesn't seem to be an obvious way to detect a submodule change.  If the TAG was put inside the `gitmodules` file that would be another thing, but it isn't.  So after a new submodule is added, the cache is never evicted (for instance in this PR's case, where an existing submodule is bumped to a new commit)

.github/workflows/linux-workflow.yml

@@ -72,18 +72,6 @@ jobs:
       - name: Checkout Repository
         uses: actions/checkout@v2
 
-      - name: Cache Submodules
-        id: cache-submodules
-        uses: actions/cache@v2.1.4
-        with:
-          key: submodules-${{ hashFiles('./.gitmodules') }}
-          path: |
-            ./.git/modules/
-            ./3rdparty/fmt
-            ./3rdparty/xz
-            ./3rdparty/yaml-cpp
-            ./3rdparty/gtest
-
       - name: Checkout Submodules
         if: steps.cache-submodules.outputs.cache-hit != 'true'
         run: git submodule update --init --recursive --jobs 2

.github/workflows/windows-workflow.yml

@@ -51,18 +51,6 @@ jobs:
       - name: Checkout Repository
         uses: actions/checkout@v2
 
-      - name: Cache Submodules
-        id: cache-submodules
-        uses: actions/cache@v2.1.4
-        with:
-          key: submodules-${{ hashFiles('./.gitmodules') }}
-          path: |
-            ./.git/modules/
-            ./3rdparty/fmt
-            ./3rdparty/xz
-            ./3rdparty/yaml-cpp
-            ./3rdparty/gtest
-
       - name: Checkout Submodules
         if: steps.cache-submodules.outputs.cache-hit != 'true'
         run: git submodule update --init --recursive --jobs 2