Remove assumeutxo / UTXO-snapshot feature

Removes the dumptxoutset RPC, -loadutxosnapshot / -loadutxosnapshotunsafe,
the CCoinsViewDB Dump/LoadSnapshot machinery + CUTXOSnapshotHeader, the
AssumeutxoData chainparams anchor, the LoadSnapshotChainstate activation +
reorg-below-H guard, the persisted assumeutxo-height flag, and the gtest.

Rationale: it duplicated the existing bootstrap (same skip-the-genesis-grind
fast-sync, no speed advantage), its only real edge was a trust model we don't
need for this chain, and it was inert anyway (no published snapshot hash in
chainparams). The -loadutxosnapshot load path adopted an external UTXO set and
bypassed genesis validation, so removing it also drops that attack surface.
Builds clean (no dangling references); the kept IBD speedups (RandomX
pre-verify, adaptive dbcache, tlsmanager) are untouched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/Makefile.gtest.include

@@ -7,11 +7,10 @@ bin_PROGRAMS += hush-gtest
 # NOTE: the original test list used an invalid automake form (comment after a trailing
 # backslash, and `zcash_gtest_SOURCES +=` with no prior `=`), which is why the whole
 # gtest harness was disabled via a `#include`. Minimal valid set: the harness + the
-# UTXO-snapshot round-trip test. Re-add other gtest sources here as they are revived.
+# Re-add other gtest sources here as they are revived.
 hush_gtest_SOURCES = \
 	gtest/main.cpp \
 	gtest/utils.cpp \
-	gtest/test_utxosnapshot.cpp \
 	gtest/test_randomx_preverify.cpp
 
 hush_gtest_CPPFLAGS = $(AM_CPPFLAGS) -DMULTICORE -fopenmp -DBINARY_OUTPUT -DCURVE_ALT_BN128 -DSTATIC $(BITCOIN_INCLUDES)