pkgbase: Remove package name mangling in generate-ucl.sh

The mangling was present in the initial revision of the script, but its
purpose is not clear. It may have been to avoid defining make(1)
variables with a dash in the name, but this is permitted. Furthermore,
it results in invalid dependency information if a dependency's name
contains an underscore, causing e.g., libcompiler_rt-dev to depend on
libcompiler-rt, and resulting in warnings when installing base system
packages. Remove the mangling.

Reviewed by: manu
MFC after: 2 months
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29883