validationcaches: Abolish arbitrary limit

1. -maxsigcachesize is a DEBUG_ONLY option

2. Almost 7 years has passed since its semantics change in
   830e3f3d02 from "number of entries" to
   "number of mebibytes"

3. A std::new_handler was added to the codebase after the original PR
   which introduced this limit, which will terminate immediately instead
   of causing trouble by being caught somewhere unexpected.

src/script/sigcache.h

@@ -16,8 +16,6 @@
 // systems). Due to how we count cache size, actual memory usage is slightly
 // more (~32.25 MB)
 static const unsigned int DEFAULT_MAX_SIG_CACHE_SIZE = 32;
-// Maximum sig cache size allowed
-static const int64_t MAX_MAX_SIG_CACHE_SIZE = 16384;
 
 class CPubKey;