Remove //net from GN exec_script whitelist.

This used to use gypi_to_gn to read sources. Since that has been removed, this whitelist entry is no longer needed.

Clarify the comment about gypi_to_gn usage: it's no longer the main reason for a whitelist entry.

Review-Url: https://codereview.chromium.org/2701573002
Cr-Commit-Position: refs/heads/master@{#450842}

.gn

@@ -195,10 +195,10 @@ check_targets = [
 #
 # GYPI_TO_GN
 #
-# Most of these entries are for gypi_to_gn calls. We should not be adding new
-# calls to this script in the build (see //build/gypi_to_gn.py for detailed
-# advice). The only time you should be editing this list for gypi_to_gn
-# purposes is when moving an existing call to a different place.
+# Some of these entries are for legacy gypi_to_gn calls. We should not be
+# adding new calls to this script in the build (see //build/gypi_to_gn.py for
+# detailed advice). The only time you should be editing this list for
+# gypi_to_gn purposes is when moving an existing call to a different place.
 #
 # PLEASE READ
 #
@@ -267,7 +267,6 @@ exec_script_whitelist =
       "//clank/native/BUILD.gn",
 
       "//jingle/BUILD.gn",
-      "//net/BUILD.gn",
       "//remoting/host/installer/linux/BUILD.gn",
       "//remoting/remoting_version.gni",
       "//remoting/host/installer/win/generate_clsids.gni",