releasetools: Remove dead parameter in GetBootableImage().

The 'system_img_path' parameter was introduced in commit
d995f4b04df46d9e9ac1be8d58ca961e94b783d1, but became obsolete since
commit 2ce63edab715ae4ff34e1a2ef240b05216f6ca5d.

Test: m dist
Change-Id: Iffd496d929db5cc3dfc955a48bfc1b1317bd012f
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 813b2c6..79b218f 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -539,7 +539,7 @@
 
 
 def GetBootableImage(name, prebuilt_name, unpack_dir, tree_subdir,
-                     info_dict=None, system_img_path=None):
+                     info_dict=None):
   """Return a File object with the desired bootable image.
 
   Look for it in 'unpack_dir'/BOOTABLE_IMAGES under the name 'prebuilt_name',