Remove check for including Runtime due to CmakeConfig.h dependencies

[lanza]

Various files include Runtime/Config.h which depends on
Runtime/CMakeConfig.h and thus this check does not work.

[lanza]

@compnerd did you do this yet? Just hit this on Darwin

[compnerd]

LGTM; this is for the include files only, and the subdirectory only generates the configuration, this won't build the runtime

[compnerd]

CC: @gparker42

[compnerd]

@swift-ci please test and merge

[gparker42]

@lanza What were you building that saw this fail?

[lanza]

@gparker42 Working with @compnerd and building stdlib-less. The demangler imports swift/runtime/Config.h which imports swift/runtime/CMakeConfig.h.

[compnerd]

@gparker42 - btw, you can now reproduce my setup pretty easily: https://github.com/compnerd/toolchain-infrastructure ... basically, I build just the compiler, then use that compiler to build the standard library in a separate build.