Remove roxygen2 messages caused by links to internal functions

[jdblischak]

xref: #351, Merck/gsDesign2#601, #227

After migrating to roxygen2 7.3.3, documenting the package produces the following verbose messages:

══ Documenting ═════════════════════════════════════════════════════════════════
ℹ Updating simtrial documentation
ℹ Loading simtrial
✖ pvalue_maxcombo.R:21: @description Could not resolve link to topic
  "fh_weight" in the dependencies or base packages.
ℹ If you haven't documented "fh_weight" yet, or just changed its name, this is
  normal. Once "fh_weight" is documented, this warning goes away.
ℹ Make sure that the name of the topic is spelled correctly.
ℹ Always list the linked package as a dependency.
ℹ Alternatively, you can fully qualify the link with a package name.
✖ pvalue_maxcombo.R:27: @param Could not resolve link to topic "fh_weight" in
  the dependencies or base packages.
ℹ If you haven't documented "fh_weight" yet, or just changed its name, this is
  normal. Once "fh_weight" is documented, this warning goes away.
ℹ Make sure that the name of the topic is spelled correctly.
ℹ Always list the linked package as a dependency.
ℹ Alternatively, you can fully qualify the link with a package name.

Both pvalue_maxcombo() and fh_weight() are internal functions with the tag @noRd, and thus the roxygen2 cross-reference syntax is no longer useful (they were exported in the past but then un-exported). I removed the cross-references so that the distracting messages are no longer produced.