remove --disable-threads & ICU_USE_THREADS

Deleted Component: config_build

• Fewer build options = easier to test/ship/support ICU (would remove one little release task)

• Using ICU in a single-threaded environment is ''slightly'' more efficient if ICU never attempts to lock mutexes etc.

• It is easier to port ICU4C to a new platform if you need not support threading, but even cell phones have multithreaded OSes now...

• Simplify mutex code

• Users could use existing API to override mutex functions with dummies