Kernel: Stop eagerly loading entire executables

We were forced to do this because the page fault code would fall apart
when trying to generate a backtrace for a non-current thread.

This issue has been fixed for a while now, so let's go back to lazily
loading executable pages which should make everything a little better.

Kernel/Makefile

@@ -109,7 +109,6 @@ CXXFLAGS += -nostdlib -nostdinc -nostdinc++
 CXXFLAGS += -I../Toolchain/Local/i686-pc-serenity/include/c++/8.3.0/
 CXXFLAGS += -I../Toolchain/Local/i686-pc-serenity/include/c++/8.3.0/i686-pc-serenity/
 DEFINES += -DKERNEL
-DEFINES += -DEXPENSIVE_USERSPACE_STACKS
 LDFLAGS += -Ttext 0x10000 -Wl,-T linker.ld -nostdlib
 
 all: $(KERNEL) kernel.map

Kernel/Process.cpp

@@ -361,10 +361,6 @@ int Process::do_exec(String path, Vector<String> arguments, Vector<String> envir
         ASSERT(success);
     }
 
-#ifdef EXPENSIVE_USERSPACE_STACKS
-    region->page_in();
-#endif
-
     OwnPtr<ELFLoader> loader;
     {
         // Okay, here comes the sleight of hand, pay close attention..

Kernel/Thread.cpp

@@ -595,11 +595,9 @@ String Thread::backtrace_impl() const
         if (!symbol.address)
             break;
         if (!symbol.ksym) {
-#ifdef EXPENSIVE_USERSPACE_STACKS
             if (!Scheduler::is_active() && process.elf_loader() && process.elf_loader()->has_symbols())
                 builder.appendf("%p  %s\n", symbol.address, process.elf_loader()->symbolicate(symbol.address).characters());
             else
-#endif
                 builder.appendf("%p\n", symbol.address);
             continue;
         }