Remove JSBackend from config.toml

JSBackend is implied when building the emscripten backend, and not available for the standard llvm backend.  This commit also puts the example config in sync with the defaults in src/bootstrap/native.rs

Author: ThisIsADogHello

config.toml.example

@@ -61,7 +61,7 @@
 # support. You'll need to write a target specification at least, and most
 # likely, teach rustc about the C ABI of the target. Get in touch with the
 # Rust team and file an issue if you need assistance in porting!
-#targets = "X86;ARM;AArch64;Mips;PowerPC;SystemZ;JSBackend;MSP430;Sparc;NVPTX;Hexagon"
+#targets = "X86;ARM;AArch64;Mips;PowerPC;SystemZ;MSP430;Sparc;NVPTX;Hexagon"
 
 # LLVM experimental targets to build support for. These targets are specified in
 # the same format as above, but since these targets are experimental, they are