Remove unused CIBW_BUILD_REQUIRES environment variable

Author: molinav

.github/workflows/build.yml

@@ -100,7 +100,6 @@ jobs:
           CIBW_SKIP: "pp* *-musllinux_* *-win32 *-manylinux_i686 *-musllinux_i686 *-aarch64 *-armv7l"
           CIBW_BEFORE_ALL: ${{ matrix.before_all }}
           CIBW_ENVIRONMENT: ${{ matrix.environment }}
-          CIBW_BUILD_REQUIRES: "cython>=0.29,<3.1"
         with:
           package-dir: packages/basemap