Some unset calls were added by commit 079b8e29 (Clang: prefer lld-link
over link.exe, 2019-07-11, v3.16.0-rc1~161^2~2) but they are unnecessary
since commit 3a82ef78

 (CMakeFindBinUtils: Rename and unset variables
for additional names, 2019-07-23, v3.16.0-rc1~332^2~2).

Suggested-by: Norbert Lange <nolange79@gmail.com>