Make the BE docgen's code a bit nicer.

Remove unused section (Generating rules) and simplify the code that
reads basic rule definitions (name, type, family) in for example:
<!-- #BLAZE_RULE (NAME = cc_binary, TYPE = BINARY, FAMILY = C / C++) -->

--
MOS_MIGRATED_REVID=88200584
diff --git a/src/main/java/com/google/devtools/build/docgen/DocgenConsts.java b/src/main/java/com/google/devtools/build/docgen/DocgenConsts.java
index e1edf29..01bf7d6 100644
--- a/src/main/java/com/google/devtools/build/docgen/DocgenConsts.java
+++ b/src/main/java/com/google/devtools/build/docgen/DocgenConsts.java
@@ -39,7 +39,6 @@
   public static final String VAR_SECTION_BINARY = "SECTION_BINARY";
   public static final String VAR_SECTION_LIBRARY = "SECTION_LIBRARY";
   public static final String VAR_SECTION_TEST = "SECTION_TEST";
-  public static final String VAR_SECTION_GENERATE = "SECTION_GENERATE";
   public static final String VAR_SECTION_OTHER = "SECTION_OTHER";
 
   public static final String VAR_IMPLICIT_OUTPUTS = "IMPLICIT_OUTPUTS";
@@ -133,6 +132,7 @@
   public static final Pattern BLAZE_RULE_ATTR_END = Pattern.compile(
       "^[\\s]*\\<!\\-\\-[\\s]*#END_BLAZE_RULE\\.ATTRIBUTE[\\s]*\\-\\-\\>[\\s]*\\*/");
 
+  /** e.g. "[DEPRECATED]" in &lt;!-- #BLAZE_RULE(...).ATTRIBUTE(...)[DEPRECATED] --&gt; */
   public static final Pattern BLAZE_RULE_FLAGS = Pattern.compile("^.*\\[(.*)\\].*$");
 
   public static final Map<String, Integer> ATTRIBUTE_ORDERING = ImmutableMap