[llvm-remarkutil] filter: Fix curses.h namespace pollution (#160802)

Remove the filter namespace, because `filter` is used by `curses.h`,
causing some external build failures

(#159784 (comment)).

We don't really need the namespace here anyways, because everything is
static. This was just following what some of the other commands in
llvm-remarkutil are doing.

Author: tobias-stadler

llvm/tools/llvm-remarkutil/RemarkFilter.cpp

@@ -20,7 +20,9 @@ using namespace llvm;
 using namespace remarks;
 using namespace llvm::remarkutil;
 
-namespace filter {
+// Note: Avoid using the identifier "filter" in this file, as it is prone to
+// namespace collision with headers that might get included e.g.
+// curses.h.
 
 static cl::SubCommand FilterSub("filter",
                                 "Filter remarks based on specified criteria.");
@@ -80,5 +82,3 @@ static Error tryFilter() {
 }
 
 static CommandRegistration FilterReg(&FilterSub, tryFilter);
-
-} // namespace filter