Remove OpenMP and No threading backends.

Since c++11, we can depend on C++ threads always being available.
With the recent work on the performance of CXX threading, the
additional complexity of maintaining multiple backends for some
minor performance delta is not worth it

https://github.com/ceres-solver/ceres-solver/issues/886

Change-Id: Idee480b22a498daec9c4366da8589aa58eaf36a1

.github/workflows/linux.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   build:
-    name: ${{matrix.os}}-${{matrix.build_type}}-${{matrix.lib}}-${{matrix.threading_model}}
+    name: ${{matrix.os}}-${{matrix.build_type}}-${{matrix.lib}}
     runs-on: ubuntu-latest
     container: ${{matrix.os}}
     defaults:
@@ -25,10 +25,6 @@ jobs:
         lib:
           - shared
           - static
-        threading_model:
-          - CXX_THREADS
-          - NO_THREADS
-          - OPENMP
 
     steps:
       - uses: actions/checkout@v2
@@ -55,8 +51,8 @@ jobs:
         uses: actions/cache@v2
         with:
           path: ${{env.CCACHE_DIR}}
-          key: ${{matrix.os}}-ccache-${{matrix.build_type}}-${{matrix.lib}}-${{matrix.threading_model}}-${{github.run_id}}
-          restore-keys: ${{matrix.os}}-ccache-${{matrix.build_type}}-${{matrix.lib}}-${{matrix.threading_model}}-
+          key: ${{matrix.os}}-ccache-${{matrix.build_type}}-${{matrix.lib}}-${{github.run_id}}
+          restore-keys: ${{matrix.os}}-ccache-${{matrix.build_type}}-${{matrix.lib}}-
 
       - name: Setup Environment
         if: matrix.build_type == 'Release'
@@ -67,7 +63,6 @@ jobs:
         run: |
           cmake -S . -B build_${{matrix.build_type}} \
                 -DBUILD_SHARED_LIBS=${{matrix.lib == 'shared'}} \
-                -DCERES_THREADING_MODEL=${{matrix.threading_model}} \
                 -DCMAKE_BUILD_TYPE=${{matrix.build_type}} \
                 -DCMAKE_C_COMPILER_LAUNCHER=$(which ccache) \
                 -DCMAKE_CXX_COMPILER_LAUNCHER=$(which ccache) \