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.

The file from https://github.com/sbt/sbt-launcher-package is changed. And the function “runAlternateBoot" is deleted in upstream project. I think spark project should delete this function in file sbt/sbt-launch-lib.bash. Thanks.

Author: KaiXinXiaoLei <huleilei1@huawei.com>

Closes #3224 from KaiXinXiaoLei/deleteFunction and squashes the following commits:

e8eac49 [KaiXinXiaoLei] Delete blank lines.
efe36d4 [KaiXinXiaoLei] Delete unnecessary function
[SPARK-4643] [Build] Remove unneeded staging repositories from build

The old location will return a 404.

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #3504 from adrian-wang/repo and squashes the following commits:

f604e05 [Daoyuan Wang] already in maven central, remove at all
f494fac [Daoyuan Wang] spark staging repo outdated
[SPARK-4193][BUILD] Disable doclint in Java 8 to prevent from build error.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #3058 from ueshin/issues/SPARK-4193 and squashes the following commits:

e096bb1 [Takuya UESHIN] Add a plugin declaration to pluginManagement.
6762ec2 [Takuya UESHIN] Fix usage of -Xdoclint javadoc option.
fdb280a [Takuya UESHIN] Fix Javadoc errors.
4745f3c [Takuya UESHIN] Merge branch 'master' into issues/SPARK-4193
923e2f0 [Takuya UESHIN] Use doclint option `-missing` instead of `none`.
30d6718 [Takuya UESHIN] Fix Javadoc errors.
b548017 [Takuya UESHIN] Disable doclint in Java 8 to prevent from build error.