signals: remove boost includes where possible

In both of these cases, boost was only needed for signals and not for
multi_index/test.

src/CMakeLists.txt

@@ -143,7 +143,6 @@ target_link_libraries(bitcoin_common
     bitcoin_util
     univalue
     secp256k1
-    Boost::headers
     $<TARGET_NAME_IF_EXISTS:USDT::headers>
     $<$<PLATFORM_ID:Windows>:ws2_32>
 )
@@ -166,7 +165,6 @@ if(ENABLE_WALLET)
       bitcoin_wallet
       bitcoin_common
       bitcoin_util
-      Boost::headers
     )
     install_binary_component(bitcoin-wallet HAS_MANPAGE)
   endif()