build: remove boost dep from libmultiprocess

Looks like this hasn't been needed since
bitcoin-core/libmultiprocess#25 and was just
missed in bitcoin#19160.

Author: fanquake

depends/packages/libmultiprocess.mk

@@ -3,7 +3,7 @@ $(package)_version=$(native_$(package)_version)
 $(package)_download_path=$(native_$(package)_download_path)
 $(package)_file_name=$(native_$(package)_file_name)
 $(package)_sha256_hash=$(native_$(package)_sha256_hash)
-$(package)_dependencies=native_$(package) boost capnp
+$(package)_dependencies=native_$(package) capnp
 
 define $(package)_config_cmds
   $($(package)_cmake)