Remove BUILD_DATETIME from Make.

Soong_UI will handle build timestamp and export date file to Make.

Test: m -j32
Bug: b/70351683
Change-Id: Ibe4ceb1b224c5e8f3e1a8a59cfee89217765a901
diff --git a/core/main.mk b/core/main.mk
index 4d43295..0c165ca 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -60,8 +60,7 @@
 # without changing the command line every time.  Avoids rebuilds
 # when using ninja.
 $(shell mkdir -p $(OUT_DIR) && \
-    echo -n $(BUILD_NUMBER) > $(OUT_DIR)/build_number.txt && \
-    echo -n $(BUILD_DATETIME) > $(OUT_DIR)/build_date.txt)
+    echo -n $(BUILD_NUMBER) > $(OUT_DIR)/build_number.txt)
 ifeq ($(HOST_OS),darwin)
 DATE_FROM_FILE := date -r $(BUILD_DATETIME_FROM_FILE)
 else