SCons: Remove unnecessary $LINK overrides

As of SCons 4.0.1, the default value for $LINK is $SMARTLINK, which itself
is a function that will use $CXX as linker for C++:

https://github.com/SCons/scons/blob/4.0.1/SCons/Tool/link.py#L327-L328
https://github.com/SCons/scons/blob/4.0.1/SCons/Tool/link.py#L54-L76

So we don't need to manually specify the same value as $CXX for $LINK.

(cherry picked from commit 2e4bff1cfe)

platform/javascript/detect.py

@@ -107,7 +107,6 @@ def configure(env):
 
     env["CC"] = "emcc"
     env["CXX"] = "em++"
-    env["LINK"] = "emcc"
 
     env["AR"] = "emar"
     env["RANLIB"] = "emranlib"