FindGLUT: Drop the now-unnecessary ALLOW_SYSTEM_CFLAGS logic

Revert commit 8041ca5df0 (FindGLUT: Fix GLUT_INCLUDE_DIRS with
pkg-config and /usr/include, 2022-05-11, v3.24.0-rc1~151^2).
As the main code path will always do `find_path()` which respects the
`CMAKE_FIND_ROOT_PATH_MODE_INCLUDE` variable and will search in system
paths depending on that variable.

Issue: #23474, #24028