target_link_libraries: Remove likely-broken ancient compatibility check

Since commit daa6d2bc04 (ENH: updated handling of debug and optimized
target link libraries, 2006-11-29, v2.6.0~2471) the ancient
`<lib>_LINK_TYPE` compatibility lookup was done using the name of the
dependent target for which `target_link_libraries` is called, rather
than the name of the library dependency being considered.  This code
probably does nothing.  Remove it.