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/config.mk b/core/config.mk
index 64743e0..911e0bf 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -846,7 +846,7 @@
 endif
 
 BUILD_NUMBER_FROM_FILE := $$(cat $(OUT_DIR)/build_number.txt)
-BUILD_DATETIME_FROM_FILE := $$(cat $(OUT_DIR)/build_date.txt)
+BUILD_DATETIME_FROM_FILE := $$(cat $(BUILD_DATETIME_FILE))
 
 # SEPolicy versions