opengl: Makefile.am: remove emscripten specific gles2 libraries

75b62f44 aims to generalize
HAVE_GLES2 with the introduction of (!HAVE_GL and HAVE_GLES2)
and removes android specific library adding.

However, emscripen, like android falls into this criteria but not
removed. Hence libraries like libvlc_opengles.la is added twice and
causing error: "duplicate symbol" during shared module building.

Fix this by removing emscripten's block of code that causes this.

modules/video_output/opengl/Makefile.am

@@ -139,12 +139,6 @@ libglfilter_mock_plugin_la_LIBADD += libvlc_opengl.la $(GL_LIBS)
 noinst_LTLIBRARIES += libglfilter_mock_plugin.la
 endif
 
-if HAVE_EMSCRIPTEN
-libglfilter_mock_plugin_la_LIBADD += libvlc_opengles.la $(GL_LIBS)
-libglfilter_mock_plugin_la_CFLAGS = -DUSE_OPENGL_ES2=1
-noinst_LTLIBRARIES += libglfilter_mock_plugin.la
-endif
-
 if HAVE_IOS_OR_TVOS
 libglfilter_mock_plugin_la_LIBADD += libvlc_opengles.la $(GLES2_LIBS)
 libglfilter_mock_plugin_la_CFLAGS = -DUSE_OPENGL_ES2=1