Removing local image lookup from load_buf

The initial logic for adding this in was incorrect. This changed had
allowed us to do fastboot flash dts dt.img on command line. But really
we should enforce that we pass in the path to the image. This change was
added so the command line tool with resemble fastboot-info format which
will support flash dts dt.img (situation where image name and partition
name don't match).

Test: fastboot flash {partition}, fastboot flashall
Bug: 194686221
Change-Id: I62f170e14860d865453ab52793e346fe4066c6d4