Remove unstable --pretty flag

It doesn't do anything `--unpretty` doesn't, and due to a bug, also
didn't show up in `--help`. I don't think there's any reason to keep it
around, I haven't seen anyone using it.

src/test/pretty/block-comment-wchar.pp

@@ -1,6 +1,6 @@
 // This is meant as a test case for Issue 3961.
 //
-// Test via: rustc --pretty normal src/test/pretty/block-comment-wchar.rs
+// Test via: rustc -Zunpretty normal src/test/pretty/block-comment-wchar.rs
 // ignore-tidy-cr
 // ignore-tidy-tab
 // pp-exact:block-comment-wchar.pp

src/test/pretty/block-comment-wchar.rs

@@ -1,6 +1,6 @@
 // This is meant as a test case for Issue 3961.
 //
-// Test via: rustc --pretty normal src/test/pretty/block-comment-wchar.rs
+// Test via: rustc -Zunpretty normal src/test/pretty/block-comment-wchar.rs
 // ignore-tidy-cr
 // ignore-tidy-tab
 // pp-exact:block-comment-wchar.pp

src/test/run-make-fulldeps/pretty-expanded/Makefile

@@ -1,5 +1,4 @@
 -include ../tools.mk
 
 all:
-	$(RUSTC) -o $(TMPDIR)/input.expanded.rs -Z unstable-options \
-		--pretty=expanded input.rs
+	$(RUSTC) -o $(TMPDIR)/input.expanded.rs -Zunpretty=expanded input.rs

src/test/run-make-fulldeps/pretty-print-to-file/Makefile

@@ -1,5 +1,5 @@
 -include ../tools.mk
 
 all:
-	$(RUSTC) -o $(TMPDIR)/input.out --pretty=normal -Z unstable-options input.rs
+	$(RUSTC) -o $(TMPDIR)/input.out -Zunpretty=normal input.rs
 	diff -u $(TMPDIR)/input.out input.pp