Replace use of boost::trim_right with locale-independent TrimString

Note the only use of readStdin is fed to DecodeHexTx, which fails in
IsHex on non-hex characters as recorded in p_util_hexdigit.

test/lint/lint-locale-dependence.sh

@@ -39,7 +39,6 @@ export LC_ALL=C
 
 KNOWN_VIOLATIONS=(
     "src/bitcoin-tx.cpp.*stoul"
-    "src/bitcoin-tx.cpp.*trim_right"
     "src/dbwrapper.cpp.*stoul"
     "src/dbwrapper.cpp:.*vsnprintf"
     "src/node/blockstorage.cpp:.*atoi"