Fix miscellaneous compiler warnings

Remove unused imports, remove unused local variables, parameterize
generic types, suppress warnings.

Change-Id: I8e4867d9a03ff774c1f8e04ff9853b0afb676e69
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-httpd/src/main/java/com/google/gerrit/httpd/WebSessionManager.java

@@ -99,7 +99,6 @@ class WebSessionManager {
       // If we don't yet have a token for this session, establish one.
       //
       final int nonceLen = 20;
-      final ByteArrayOutputStream buf;
       final byte[] rnd = new byte[nonceLen];
       prng.nextBytes(rnd);
       xsrfToken = CookieBase64.encode(rnd);