Remove mention of "compiler-rt" from COPYRIGHT file

The compiler-rt dependency was removed in
7e6c9f363501c49d3a1f666d85d41891f50890b8 in favor of a vendored
dependency on rust-lang/compiler-builtins (dual UIUC and MIT licensed).
That vendored dependency was converted to a regular Cargo dependency in
4c21a3bc2afc5933f31f1368f6b3406c5f1bdeb3.