[buildsystem] Disable search for Boost in parser detection

This changeset disables the effect of commit e9a5e82.  There appears
to be an unforeseen interaction between the calls to

    Find_Package (Boost)

that are implied by module 'opm-parser-prereqs.cmake' and the one
that's directly invoked in 'Findopm-parser.cmake'.  Until the
situation can be fully analysed and a complete solution implemented,
this is a temporary measure to restore the build of opm-core and
downstream OPM modules.
Merge pull request #600 from bska/restore-build

[buildsystem] Disable search for Boost in parser detection