[infra/debian] Remove unused import-statement from test script

Fixes debian builder broken by https://dart.googlesource.com/sdk/+/77446204dac5eae6f8407e371873612359f8815c

Change-Id: I87f7174d11a88070897a406181169f051addc877
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/291720
Commit-Queue: Alexander Aprelev <aam@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

tools/linux_dist_support/linux_distribution_support.py

@@ -36,7 +36,6 @@ def UninstallDart():
 def CreateDartTestFile(tempdir):
     filename = os.path.join(tempdir, 'test.dart')
     with open(filename, 'w') as f:
-        f.write('import "dart:collection";\n\n')
         f.write('void main() {\n')
         f.write('  print("Hello world");\n')
         f.write('}')