Remove gmp as a dependency since we don't actually use it

Long ago our code actually used gmp directly but that is no longer the case.
Our dependency secp256k1 has an optional dependency on gmp but the way we
configure and compile secp256k1 does not use it. So there is no reason
to download and compile gmp in our build system. This means we no longer
need to maintain three different forks of GMP (one for Darwin, one for Win and
one for Linux that supports gcc15). This also makes downloading and compiling
Hush faster. Joy to the world.

depends/packages/libgmp.mk

@@ -1,52 +0,0 @@
-package=libgmp
-
-
-ifeq ($(host_os),mingw32)
-$(package)_download_path=https://github.com/radix42/$(package)/archive
-$(package)_file_name=$(package)-$($(package)_git_commit).tar.gz
-$(package)_download_file=$($(package)_git_commit).tar.gz
-$(package)_sha256_hash=67df06ed50f288bd7b1ec6907973684fb7cf1196f2cb368b59d423e42b065e40
-$(package)_git_commit=42ba95387cdfd67399f7aac52fddb8d6e1258ee6
-$(package)_dependencies=
-$(package)_config_opts=--enable-cxx --disable-shared
-else
-#else ifeq ($(build_os),darwin)
-$(package)_version=6.1.1
-$(package)_download_path=https://git.hush.is/attachments
-$(package)_file_name=d613c855-cd92-4efb-b893-658496852019
-$(package)_download_file=d613c855-cd92-4efb-b893-658496852019
-$(package)_sha256_hash=a8109865f2893f1373b0a8ed5ff7429de8db696fc451b1036bd7bdf95bbeffd6
-$(package)_config_opts=--enable-cxx --disable-shared
-$(package)_patches=gcc-15.patch
-endif
-
-#else
-#$(package)_version=6.1.1
-#$(package)_download_path=https://ftp.gnu.org/gnu/gmp
-#$(package)_file_name=gmp-$($(package)_version).tar.bz2
-#$(package)_sha256_hash=a8109865f2893f1373b0a8ed5ff7429de8db696fc451b1036bd7bdf95bbeffd6
-#$(package)_dependencies=
-#$(package)_config_opts=--enable-cxx --disable-shared
-#endif
-
-define $(package)_preprocess_cmds
-  patch -p1 < $($(package)_patch_dir)/gcc-15.patch
-endef
-
-define $(package)_config_cmds
-  $($(package)_autoconf) --host=$(host) --build=$(build)
-endef
-
-ifeq ($(build_os),darwin)
-define $(package)_build_cmds
-	$(MAKE)
-endef
-else
-define $(package)_build_cmds
-  $(MAKE) CPPFLAGS='-fPIC'
-endef
-endif
-
-define $(package)_stage_cmds
-  $(MAKE) DESTDIR=$($(package)_staging_dir) install ; echo '=== staging find for $(package):' ; find $($(package)_staging_dir)
-endef

depends/packages/packages.mk

@@ -1,4 +1,4 @@
-zcash_packages :=  libgmp libsodium utfcpp
+zcash_packages :=  libsodium utfcpp
 rust_crates := \
   crate_aes \
   crate_aesni \