file has been redundant since r2278.

include/innodb_redefine.h: Remove.

include/sync0sync.h: Remove the definition of mutex_free that was added
because of innodb_redefine.h.

include/univ.i: Remove references to innodb_redefine.h.  Explain why
the C++ classes need to be renamed.

scripts/build-plugin.sh: Build InnoDB only once.  The file
innodb_redefine.h is no longer needed.