Revert "wasm: support emsdk >= 1.39.4"

This reverts commit bcdf49bcc6c357c9c30708e2e95ce4b8dbeb9f8a.

This was a 5.14-only commit which accidentally found its
way to 5.15.

Change-Id: Iaef6f05fce20ab61d3afb8cb3c359a037e4c0b28
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

mkspecs/wasm-emscripten/qmake.conf

@@ -37,7 +37,6 @@ EMCC_COMMON_LFLAGS += \
     -s NO_EXIT_RUNTIME=0 \
     -s ERROR_ON_UNDEFINED_SYMBOLS=1 \
     -s EXTRA_EXPORTED_RUNTIME_METHODS=[\"UTF16ToString\",\"stringToUTF16\"] \
-    -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=0 \
     --bind
 
 # The -s arguments can also be used with release builds,