[dev.link] cmd/link/internal/loader: remove Propagate* shim functions

[dev.link] cmd/link/internal/loader: remove Propagate* shim functions

Remove the loader's PropagateSymbolChangesBackToLoader and
PropagateLoaderChangesToSymbols shim functions. These were used at one
point to enable conversion of phases in the linker that were
"downstream" of loadlibfull -- given the current wavefront position
there's not much point keeping them around.

Change-Id: I3f01f25b70b1b80240369c8f3a10dca89931610f
Reviewed-on: https://go-review.googlesource.com/c/go/+/233817
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>