astro/gpstk: drop unused dependencies

[jbeich]

Summary

rP370634 added USE_GCC without documenting rationale. gpstk-2.5 built fine by Clang and no consumers required libgpstk.so to be linked against libstdc++.so.

Kurt, why did you add the dependency then? To fix a runtime issue? I don't use the port, so have no clue, and there's no maintainer to ask.

Test Plan

Builds fine on

• lang/gcc7
• Clang 5.0 + libstdc++ 4.2
• 10.3 amd64 (Clang/libc++ 3.4)
• 10.3 i386 (Clang/libc++ 3.4)
• 10.4 amd64 (Clang/libc++ 3.4)
• 10.4 i386 (Clang/libc++ 3.4)
• 11.0 amd64 (Clang/libc++ 3.8)
• 11.0 i386 (Clang/libc++ 3.8)
• 11.1 aarch64 (Clang/libc++ 4.0)
• 11.1 amd64 (Clang/libc++ 4.0)
• 11.1 armv6 (Clang/libc++ 4.0)
• 11.1 i386 (Clang/libc++ 4.0)
• 11.1 mips64 (GCC/libstdc++ 4.2)
• 12.0 amd64 (Clang/libc++ 5.0)
• 12.0 i386 (Clang/libc++ 5.0)

$ pkg info -ql gpstk | xargs fgrep python
/usr/local/include/TabularSatStore.hpp:   // typedefs had to be set to public for python bindings to compile

[pi]

That was a long time ago, it probably did not build at that time without USE_GCC. If it builds now, remove USE_GCC, that's fine with me.