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
1 file changed