Remove the link_rust.sh.in script

This is no longer necessary with upstream rust-lang/rust changes as well
as some local tweaks. Namely:

* The `-fsanitize=address`-style options are now passed via `-C
  link-args` through `RUSTFLAGS`. This obviates the need for the shell
  script.
* The `-C default-linker-libraries`, disabling `-nodefaultlibs`, is
  passed through `RUSTFLAGS`, which is necessary to ensure that
  `-fsanitize=address` links correctly.
* The `-C linker` option is passed to ensure we're using the same C
  compiler as normal C code, although it has a bit of hackery to only
  get the `gcc` out of `gcc -std=c99`

src/test/include.am

@@ -12,8 +12,7 @@ TESTS_ENVIRONMENT = \
 	export EXTRA_CARGO_OPTIONS="$(EXTRA_CARGO_OPTIONS)"; \
 	export CARGO_ONLINE="$(CARGO_ONLINE)"; \
 	export CCLD="$(CCLD)"; \
-	chmod +x "$(abs_top_builddir)/link_rust.sh"; \
-	export RUSTFLAGS="-C linker=$(abs_top_builddir)/link_rust.sh";
+	export RUSTFLAGS="-C linker=`echo '$(CC)' | cut -d' ' -f 1` $(RUST_LINKER_OPTIONS) -C default-linker-libraries";
 
 TESTSCRIPTS = \
 	src/test/fuzz_static_testcases.sh \