Remove unneeded policy setting.

We shouldn't need this anymore: policy CMP0048 was introduced in CMake
3.0; we started setting it back when CMake 2.8.12 was our minimum, but
now CMake 3.5.1 is our minimum.

Change-Id: Ice64a376cef3ea1420e9d3d73de6efa5bd95e71c
Reviewed-on: https://code-review.googlesource.com/c/re2/+/58610
Reviewed-by: Paul Wankadia <junyer@google.com>