[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
```cmake
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.