Remove no_integrated_as mode.

Specifically, remove both `-Z no_integrated_as` and
`TargetOptions::no_integrated_as`. The latter was only used for the
`msp430_none_elf` platform, for which it's no longer required.

src/test/run-make-fulldeps/no-integrated-as/Makefile

@@ -1,8 +0,0 @@
--include ../tools.mk
-
-# only-linux
-# only-x86_64
-
-all:
-	$(RUSTC) hello.rs -C no_integrated_as
-	$(call RUN,hello)

src/test/run-make-fulldeps/no-integrated-as/hello.rs

@@ -1,3 +0,0 @@
-fn main() {
-    println!("Hello, world!");
-}