[SPARK-9268] [ml] Removed varargs annotation from Params.setDefault taking multiple params

[jkbradley]

Removed varargs annotation from Params.setDefault taking multiple params.

Though varargs is technically correct, it often requires that developers do clean assembly, rather than (not clean) assembly, which is a nuisance during development.

CC: @mengxr

[mengxr]

LGTM if MiMa doesn't complain.

[SparkQA]

Test build #38134 has finished for PR 7604 at commit 6016dc6.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class CreateArray(children: Seq[Expression]) extends Expression
  • case class CreateStruct(children: Seq[Expression]) extends Expression
  • case class CreateNamedStruct(children: Seq[Expression]) extends Expression

[mengxr]

Merged into master. Thanks!