CI: remove macos-10.15 jobs as the macos-10.15 runners are removed (#695

)

Since 2022-08-30 the `macos-10.15` GitHub Actions runner are removed. We
already have the recommended `macos-12` based runners in use. So just
remove the old jobs to fix the pipelines.

Fixes: #694

.github/workflows/ci/matrix.json

@@ -8,7 +8,6 @@
 { "example": "foo", "toolchain": "sanitize-address-cxx17",                          "os": "ubuntu-20.04", "python": "3.8", "script": "build.sh"  },
 { "example": "foo", "toolchain": "sanitize-leak-cxx17",                             "os": "ubuntu-20.04", "python": "3.8", "script": "build.sh"  },
 { "example": "foo", "toolchain": "sanitize-thread-cxx17",                           "os": "ubuntu-20.04", "python": "3.8", "script": "build.sh"  },
-{ "example": "foo", "toolchain": "osx-10-15-make-cxx14",                            "os": "macos-10.15",  "python": "3.8", "script": "build.sh"  },
 { "example": "foo", "toolchain": "osx-12-3-arch-universal2-cxx17",                  "os": "macos-12",     "python": "3.8", "script": "build.sh"  },
 { "example": "foo", "toolchain": "ios-nocodesign-15-5-arm64-cxx17",                 "os": "macos-12",     "python": "3.8", "script": "build.sh"  },
 { "example": "foo", "toolchain": "ninja-vs-16-2019-win64-cxx17",                    "os": "windows-2019", "python": "3.8", "script": "build.cmd" },