minor dead code: unused format string

Using URL base64 encoding
(without padding, i.e. with trailing '=' or '==')
hash string width is 256/6 = 43 characters.

file size virtually unchanged,
  out/soong/build.ninja:  7,188,669,041 => 7,186,999,370 ie 1.6M

Bug: N/A
Test: manually verified (e.g. ran `m nothing` successfully)
Change-Id: I166d613e1fd857555da9611d420d6691806571c7