Remove BuildSystemQEMU.

Reduce a number of additional abstractions required for the user mode
support that should be eventually removed once the user mode is merged
into the qemu-cheri branch.

This change uses a hack to always pass --disabled-bsd-user to configure
and later pass --enable-bsd-user to overwrite it.

Pointed out by:	jrtc27

Author: kwitaszczyk

pycheribuild/projects/build_qemu.py

@@ -189,6 +189,7 @@ def setup(self):
             "--target-list=" + self.qemu_targets,
             "--enable-slirp=git",
             "--disable-linux-user",
+            "--disable-bsd-user",
             "--disable-xen",
             "--disable-docs",
             "--disable-rdma",
@@ -226,17 +227,8 @@ def update(self):
         super().update()
 
 
-class BuildSystemQEMU(BuildQEMUBase):
-    do_not_add_to_targets = True
-
-    def setup(self):
-        super().setup()
-        # Don't build BSD user mode targets as we only want system mode binaries.
-        self.configure_args.append("--disable-bsd-user")
-
-
 # noinspection PyAbstractClass
-class BuildUpstreamQEMU(BuildSystemQEMU):
+class BuildUpstreamQEMU(BuildQEMUBase):
     repository = GitRepository("https://github.com/qemu/qemu.git")
     target = "upstream-qemu"
     _default_install_dir_fn = ComputedDefaultValue(
@@ -247,7 +239,7 @@ class BuildUpstreamQEMU(BuildSystemQEMU):
                       "x86_64-softmmu,aarch64-softmmu"
 
 
-class BuildQEMU(BuildSystemQEMU):
+class BuildQEMU(BuildQEMUBase):
     target = "qemu"
     repository = GitRepository("https://github.com/CTSRD-CHERI/qemu.git", default_branch="qemu-cheri")
     default_targets = "mips64-softmmu,mips64cheri128-softmmu," \
@@ -358,7 +350,7 @@ def setup(self):
         self.configure_args.append("--static")
 
 
-class BuildMorelloQEMU(BuildSystemQEMU):
+class BuildMorelloQEMU(BuildQEMUBase):
     repository = GitRepository("https://github.com/CTSRD-CHERI/qemu.git", default_branch="qemu-morello-merged",
                                force_branch=True,
                                old_urls=[