theme: clean up TW_THEME_VERSION shell command

Change-Id: I6be1f15d05e6ad53bf239674cf41173879eef37a
diff --git a/Android.mk b/Android.mk
index d3cd3b0..a383895 100755
--- a/Android.mk
+++ b/Android.mk
@@ -560,7 +560,7 @@
 
 LOCAL_REQUIRED_MODULES += $(TWRP_REQUIRED_MODULES)
 
-TW_THEME_VERSION := $(shell echo "`grep TW_THEME_VERSION bootable/recovery/variables.h | cut -d ' ' -f 3`")
+TW_THEME_VERSION := $(shell grep TW_THEME_VERSION bootable/recovery/variables.h | cut -d ' ' -f 3)
 
 LOCAL_POST_INSTALL_CMD += \
     sed -i "s/{themeversion}/$(TW_THEME_VERSION)/" $(TARGET_RECOVERY_ROOT_OUT)/twres/splash.xml; \