chore: get rid of dead code and unused variables where appropriate

Looks like some cruft has been left over from previous refactorings.
This removes dead variables, which should not have side effects in their
constructors. In cases where the variable initialization has a purpose
[[maybe_unused]] is inserted to silence compiler warnings.

src/libutil/serialise.cc

@@ -90,7 +90,6 @@ void Source::operator () (std::string_view data)
 
 void Source::drainInto(Sink & sink)
 {
-    std::string s;
     std::array<char, 8192> buf;
     while (true) {
         size_t n;
@@ -427,7 +426,7 @@ Error readError(Source & source)
     auto type = readString(source);
     assert(type == "Error");
     auto level = (Verbosity) readInt(source);
-    auto name = readString(source); // removed
+    [[maybe_unused]] auto name = readString(source); // removed
     auto msg = readString(source);
     ErrorInfo info {
         .level = level,

src/libutil/terminal.cc

@@ -26,7 +26,7 @@ bool isTTY()
 
 std::string filterANSIEscapes(std::string_view s, bool filterAll, unsigned int width)
 {
-    std::string t, e;
+    std::string t;
     size_t w = 0;
     auto i = s.begin();