stop attempting to suppress double '-g' with --enable-debug

stop attempting to suppress double '-g' with --enable-debug

This reverts part of 3b7da5bc0345872efcf011c9c614cc27f82b9998. It is unclear
whether this was a typo or just an insufficiently scoped attempt to clean up
build flags. Of the problems with this code, the main ones were:

  1. It seems somewhat purposeless to suppress `-g` only to just re-add it. GCC
     and ICC can both cope with duplicate `-g` options. It seems purely
     aesthetically-motivated gymnastics to try and pass the compiler a sole
     `-g`.

  2. The scope of things that matched this replacement was too wide. For
     example, a `-ggdb` option would be mangled into `gdb` which is almost
     certainly not what the user intended and would result in cryptic build
     errors. Something like `-fprofile-generate` would be mangled into
     `-fprofileenerate` resulting in similarly confusing build errors.

Related to #2115.