Delete unnecessary function

when building spark by sbt, the function “runAlternateBoot" in sbt/sbt-launch-lib.bash is not used. And this function is not used by spark code. So I think this function is not necessary. And the option of "sbt.boot.properties" can be configured in the command line when building spark, eg: 
sbt/sbt assembly -Dsbt.boot.properties=$bootpropsfile.