New review comment by kruceter on void-packages repository

https://github.com/void-linux/void-packages/pull/39583#discussion_r1059853962

Comment:
I cannot make any sense from these expressions.

It does seem that even with `make_use_env` applied, cmt's makefile ignores external variables (CFLAGS in this case) and the second expression attempts to stuff them into the file.

I would rather replace these crunches with

```sh
sed -e "/^CFLAGS/d; /^CXXFLAGS/d" -i "${build_wrksrc}/Makefile"
```

and remove `make_use_env=yes` as it does not fit its purpose here.