Remove default for --images in common flags

Review URL: https://codereview.chromium.org/1214363002
diff --git a/tools/flags/SkCommonFlags.cpp b/tools/flags/SkCommonFlags.cpp
index 09e74ba..7c7634f 100644
--- a/tools/flags/SkCommonFlags.cpp
+++ b/tools/flags/SkCommonFlags.cpp
@@ -24,7 +24,7 @@
               "Defaults to empty string, which selects the API native to the "
               "system.");
 
-DEFINE_string(images, "resources", "Directory of images to decode.");
+DEFINE_string(images, "", "Directory of images to decode.");
 
 DEFINE_string2(match, m, NULL,
                "[~][^]substring[$] [...] of GM name to run.\n"