Revert "configs: use 16GB memory as default for ref"

This reverts commit 104ae2c.

To run Xiangshan nightly CI, 16GB memory nemu so should be
manually compiled. A pre-compiled 16GB nemu is included in
ready-to-run.

configs/riscv64-xs-ref_defconfig

@@ -48,24 +48,17 @@ CONFIG_CC_DEBUG=y
 #
 # Testing and Debugging
 #
-# CONFIG_DEBUG is not set
 CONFIG_DIFFTEST_REF_PATH="none"
 CONFIG_DIFFTEST_REF_NAME="none"
 # CONFIG_DETERMINISTIC is not set
 # CONFIG_IQUEUE is not set
-# CONFIG_MEMLOG is not set
-# CONFIG_TRANSLOG is not set
-# CONFIG_EXITLOG is not set
-# CONFIG_TRACE_INST is not set
-# CONFIG_TRACE_BB is not set
 # end of Testing and Debugging
 
 #
 # Memory Configuration
 #
 CONFIG_MBASE=0x80000000
-CONFIG_MSIZE=0x400000000
-CONFIG_BBL_OFFSET_WITH_CPT=0xa0000
+CONFIG_MSIZE=0x200000000
 CONFIG_RESET_FROM_MMIO=y
 CONFIG_PC_RESET_OFFSET=0x0
 CONFIG_MMIO_RESET_VECTOR=0x10000000