Delete all code for handling lldb and gdb versions that don't support Rust natively

[saethlin]

We have a fair bit of code in both src/tools/compiletest and tests/debuginfo devoted to handling the 7.X releases of gdb, because those had no built-in Rust support. But as of this year at least, the oldest supported RHEL version is now shipping gdb 8.2 so there's no reason for us to be pretending to support versions of gdb before 8.2.

The dev guide also vaguely indicates that this distinction for lldb shouldn't exist anymore:

https://github.com/rust-lang/rustc-dev-guide/blob/43d83780db545a1ed6d45773312fc578987e3968/src/tests/compiletest.md?plain=1#L216
https://github.com/rust-lang/rustc-dev-guide/blob/43d83780db545a1ed6d45773312fc578987e3968/src/tests/compiletest.md?plain=1#L253-L255

• Delete all the gdbg annotations in tests/debuginfo, and convert all gdbr to just gdb
• Delete all the lldbg annotations in tests/debuginfo and convert all lldbr to lldb
• Check if the tests still pass (you've got a 50/50 shot on this, debuginfo tests are cursed)
• Delete the code in compiletest that handles this "Rust support" version gap for gdb, and for lldb