Remove the unstable SYNCHRONOUS_DNS_CACHE testcase

Will be reverted once the test case is fixed

Author: Seppo Takalo

TESTS/netsocket/dns/main.cpp

@@ -178,9 +178,6 @@ Case cases[] = {
 #endif
     Case("SYNCHRONOUS_DNS", SYNCHRONOUS_DNS),
     Case("SYNCHRONOUS_DNS_MULTIPLE", SYNCHRONOUS_DNS_MULTIPLE),
-#ifdef MBED_EXTENDED_TESTS
-    Case("SYNCHRONOUS_DNS_CACHE", SYNCHRONOUS_DNS_CACHE),
-#endif
     Case("SYNCHRONOUS_DNS_INVALID", SYNCHRONOUS_DNS_INVALID),
 };