[openmp] Remove __ANDROID_API__ < 19 workaround

https://github.com/android/ndk/wiki/Changelog-r24 shows that the NDK has
moved forward to at least a minimum target API of 19. Remove old workaround.

GitOrigin-RevId: acfe0d3b157361433977c3341a0ec88ebeabc2f7
1 file changed