zephyr/prj_qemu_x86.conf: Remove outdated CONFIG_RAM_SIZE.

Target RAM size is no longer set using Kconfig options, but instead using
DTS (device tree config). Fortunately, the default is now set to a high
value, so we don't need to use DTS fixup.

ports/zephyr/prj_qemu_x86.conf

@@ -5,6 +5,3 @@ CONFIG_CONSOLE_PULL=n
 # Networking drivers
 # SLIP driver for QEMU
 CONFIG_NET_SLIP_TAP=y
-
-# Default RAM easily overflows with uPy and networking
-CONFIG_RAM_SIZE=320