Remove invalid test

When ran on Windows `cp` will follow symlink: `checkout/build/<target>/<stage>/lib/rustlib/src/rust`.
It points to `checkout` which means the test will get stuck in copying loop until there is no space left.

src/test/run-make-fulldeps/libs-search-path/Makefile

@@ -1,10 +0,0 @@
--include ../tools.mk
-
-# only-mingw
-
-all: empty.rs
-	cp -r $(shell cygpath -u $(shell $(RUSTC) --print sysroot)) $(TMPDIR)/sysroot
-	$(RUSTC) --target $(TARGET) --sysroot $(TMPDIR)/sysroot -L$(TMPDIR)/obj -Z print-link-args empty.rs | $(CGREP) 'lib\\crt2.o'
-	mkdir -p $(TMPDIR)/obj
-	mv $(TMPDIR)/sysroot/lib/rustlib/$(TARGET)/lib/crt2.o $(TMPDIR)/obj/crt2.o
-	$(RUSTC) --target $(TARGET) --sysroot $(TMPDIR)/sysroot -L$(TMPDIR)/obj -Z print-link-args empty.rs | $(CGREP) 'obj\\crt2.o'

src/test/run-make-fulldeps/libs-search-path/empty.rs

@@ -1 +0,0 @@
-fn main() {}