8217329: JTREG: Clean up, remove unused imports in gc folder

[Leo Korinth]

Hi,

This fix removes unused imports and newlines in the jtreg/gc folder.

This fix is somewhat based/dependent on "8214799: Add package 
declaration to each JTREG test case in the gc folder". That fix is out 
for review.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8217329

Webrev:
http://cr.openjdk.java.net/~lkorinth/8217329/00

Testing:
passed locally: open/test/hotspot/jtreg/:hotspot_gc

Thanks,
Leo