Drop the use of `average_size` with hypothesis.strategies.lists (#14890)

It has been removed and now causes an error:
https://hypothesis.readthedocs.io/en/latest/changes.html#v4-0-0

Fixes https://github.com/web-platform-tests/wpt/issues/14889.
diff --git a/tools/manifest/tests/test_manifest.py b/tools/manifest/tests/test_manifest.py
index 010a0c0..8f649f1 100644
--- a/tools/manifest/tests/test_manifest.py
+++ b/tools/manifest/tests/test_manifest.py
@@ -66,8 +66,7 @@
 
 
 @h.given(hs.lists(sourcefile_strategy(),
-                  min_size=1, average_size=10, max_size=1000,
-                  unique_by=lambda x: x.rel_path))
+                  min_size=1, max_size=1000, unique_by=lambda x: x.rel_path))
 @h.example([SourceFileWithTest("a", "0"*40, item.ConformanceCheckerTest)])
 def test_manifest_to_json(s):
     m = manifest.Manifest()
@@ -83,8 +82,7 @@
 
 
 @h.given(hs.lists(sourcefile_strategy(),
-                  min_size=1, average_size=10,
-                  unique_by=lambda x: x.rel_path))
+                  min_size=1, unique_by=lambda x: x.rel_path))
 @h.example([SourceFileWithTest("a", "0"*40, item.TestharnessTest)])
 @h.example([SourceFileWithTest("a", "0"*40, item.RefTest, [("/aa", "==")])])
 def test_manifest_idempotent(s):