Resolve the following technical debt issues: misc-unused-using-decls

CL generated via Upkeep (go/upkeep).

#upkeep #autofix #codehealth #cleanup

Tested:
    Local presubmit tests passed.
PiperOrigin-RevId: 738030989
diff --git a/quiche/http2/hpack/decoder/hpack_decoder_string_buffer_test.cc b/quiche/http2/hpack/decoder/hpack_decoder_string_buffer_test.cc
index a54c511..5df6d42 100644
--- a/quiche/http2/hpack/decoder/hpack_decoder_string_buffer_test.cc
+++ b/quiche/http2/hpack/decoder/hpack_decoder_string_buffer_test.cc
@@ -18,7 +18,6 @@
 
 using ::testing::AssertionResult;
 using ::testing::AssertionSuccess;
-using ::testing::HasSubstr;
 
 namespace http2 {
 namespace test {
diff --git a/quiche/http2/hpack/decoder/hpack_whole_entry_buffer_test.cc b/quiche/http2/hpack/decoder/hpack_whole_entry_buffer_test.cc
index b1d3907..39d4e39 100644
--- a/quiche/http2/hpack/decoder/hpack_whole_entry_buffer_test.cc
+++ b/quiche/http2/hpack/decoder/hpack_whole_entry_buffer_test.cc
@@ -9,7 +9,6 @@
 
 #include "quiche/common/platform/api/quiche_test.h"
 
-using ::testing::_;
 using ::testing::AllOf;
 using ::testing::InSequence;
 using ::testing::Property;