Fix compiler warnings

Merge from Widevine repo of http://go/wvgerrit/43420

Remove or mark unused variables.  Fix unsigned/signed comparisons.

bug: 73390805
test: unit tests
Change-Id: Ic523400a5decf82fae733042b260e0c39a087cd3

libwvdrmengine/cdm/test/cdm_feature_test.cpp

@@ -47,12 +47,11 @@ namespace {
 
 #define N_ELEM(a) (sizeof(a) / sizeof(a[0]))
 
-const char kPathDelimiter = '/';
-
 // HTTP response codes.
 const int kHttpOk = 200;
-const int kHttpBadRequest = 400;
-const int kHttpInternalServerError = 500;
+// The following two responses are unused, but left here for human debuggers.
+// const int kHttpBadRequest = 400;
+// const int kHttpInternalServerError = 500;
 
 // Default license server, can be configured using --server command line option
 // Default key id (pssh), can be configured using --keyid command line option