Revert "Add PLATFORM key to lit.cfg"

This reverts commit 09ece8c.

We are working towards enabling cross-compilation between all platforms,
and this change crosses the signals between the host and target
platforms.

Author: gribozavr

test/lit.cfg

@@ -430,7 +430,6 @@ for target in config.targets_to_build.split():
 # Add the run target CPU, OS, and pointer size as features.
 config.available_features.add("CPU=" + run_cpu)
 config.available_features.add("OS=" + run_os)
-config.available_features.add("PLATFORM=" + platform.system())
 config.available_features.add("PTRSIZE=" + run_ptrsize)
 
 if run_cpu == "i386" or run_cpu == "x86_64":