adb: remove use of mkdirs from `adb backup`.

`adb backup -f nonexistent/directories/foo <pkg>` shouldn't create the
directory structure if they don't already exist.

Change-Id: Ifd8769ed7ee5e733a6635751ad09b714ab58ea4b

adb/commandline.cpp

@@ -950,7 +950,6 @@ static int backup(int argc, const char** argv) {
     if (argc < 2) return usage();
 
     adb_unlink(filename);
-    mkdirs(filename);
     int outFd = adb_creat(filename, 0640);
     if (outFd < 0) {
         fprintf(stderr, "adb: unable to open file %s\n", filename);