base: Get rid of the unused "debuggers" vector in the remote GDB stub.

This was presumably there for debuggers attached to gem5 itself to
examine, but doesn't provide much value and adds a small amount of
complexity.

Change-Id: Iaed08b63beafe8bf05f1496f6341a7feadc350ce
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/44030
Reviewed-by: Boris Shingarov <shingarov@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/remote_gdb.cc

@@ -155,8 +155,6 @@ static const char GDBEnd = '#';
 static const char GDBGoodP = '+';
 static const char GDBBadP = '-';
 
-std::vector<BaseRemoteGDB *> debuggers;
-
 class HardBreakpoint : public PCEvent
 {
   private:
@@ -316,7 +314,6 @@ BaseRemoteGDB::BaseRemoteGDB(System *_system, ThreadContext *c, int _port) :
         active(false), attached(false), sys(_system), tc(c),
         trapEvent(this), singleStepEvent(*this)
 {
-    debuggers.push_back(this);
 }
 
 BaseRemoteGDB::~BaseRemoteGDB()