[cmake] Remove VERSION property from executable targets

Summary:
Currently, in the default configuration, the "install" target will
install all llvm executables unversioned, except for three lldb tools
which will be installed versioned (with a non-versioned symlink). This
rectifies that situation.

Reviewers: beanz, sylvestre.ledru, mgorny

Subscribers: ki.stfu, lldb-commits

Differential Revision: https://reviews.llvm.org/D29126

llvm-svn: 293803