don't force system cJSON on Ubuntu Focal

Ubuntu Focal's cJSON does not contain a const correctness fix that
Monado requires in order to compile with clang:

https://gitlab.freedesktop.org/monado/monado/-/commit/a530b6d3750fb2762dc783a624c7e2ac5d3b2d9d

debian/control

@@ -8,7 +8,6 @@ Build-Depends: debhelper-compat (= 12),
                glslang-tools,
                libavcodec-dev [!hppa !sh4],
                libbsd-dev [linux-any],
-               libcjson-dev,
                libdbus-1-dev [linux-any],
                libegl1-mesa-dev,
                libeigen3-dev,

debian/rules

@@ -19,7 +19,7 @@ include /usr/share/dpkg/architecture.mk
 # We disable absolute systemd unit files too since we install to the path.
 # Don't waste time building the SteamVR driver, we don't package it.
 # Tell the version, instead of letting it guess from Git.
-CONFIG_ARGS := -DXRT_HAVE_SYSTEM_CJSON=ON \
+CONFIG_ARGS := \
 	  -DXRT_OPENXR_INSTALL_ACTIVE_RUNTIME=OFF \
 	  -DXRT_VULKAN_ENABLE_VALIDATION=OFF \
 	  -DBUILD_DOC=OFF \