make: remove redundant variables

The GNU make manual advises not to use variables for commands like `rm`
which are widely available on all Linux and UNIX-like platforms.

Makefile

@@ -7,26 +7,24 @@ BASHCOMPDIR ?= ${DATAROOTDIR}/bash-completion/completions
 FISHCOMPDIR ?= ${DATAROOTDIR}/fish/vendor_completions.d
 PROG ?= tessen
 
-RM := rm
-ECHO := @echo
 SCDOC := scdoc
 INSTALL := install
 
 .PHONY: all install minimal bashcomp fishcomp clean uninstall
 
 all:
-	${ECHO} "${PROG} is a shell script and doesn't need to be compiled"
-	${ECHO} "To install it, enter \"make install\""
+	@echo "${PROG} is a shell script and doesn't need to be compiled"
+	@echo "To install it, enter \"make install\""
 
 install: man bashcomp fishcomp
 	${INSTALL} -Dm 0755 ${PROG} -t ${DESTDIR}${BINDIR}
-	${ECHO} ""
-	${ECHO} "${PROG} has been installed succesfully"
+	@echo ""
+	@echo "${PROG} has been installed succesfully"
 
 minimal:
 	${INSTALL} -Dm 0755 ${PROG} -t ${DESTDIR}${BINDIR}
-	${ECHO} ""
-	${ECHO} "${PROG} has been installed succesfully"
+	@echo ""
+	@echo "${PROG} has been installed succesfully"
 
 man: man/${PROG}.1 man/${PROG}.5
 	${INSTALL} -Dm 0644 man/${PROG}.1 -t ${DESTDIR}${MANDIR}/man1
@@ -42,12 +40,12 @@ fishcomp:
 	${INSTALL} -Dm 0644 completion/${PROG}.fish-completion ${DESTDIR}${FISHCOMPDIR}/${PROG}.fish
 
 clean:
-	${RM} -f man/${PROG}.1
-	${RM} -f man/${PROG}.5
+	rm -f man/${PROG}.1
+	rm -f man/${PROG}.5
 
 uninstall:
-	${RM} -f "${DESTDIR}${BINDIR}/${PROG}"
-	${RM} -f "${DESTDIR}${MANDIR}/man1/${PROG}.1"
-	${RM} -f "${DESTDIR}${MANDIR}/man5/${PROG}.5"
-	${RM} -f "${DESTDIR}${BASHCOMPDIR}/${PROG}"
-	${RM} -f "${DESTDIR}${FISHCOMPDIR}/${PROG}.fish"
+	rm -f "${DESTDIR}${BINDIR}/${PROG}"
+	rm -f "${DESTDIR}${MANDIR}/man1/${PROG}.1"
+	rm -f "${DESTDIR}${MANDIR}/man5/${PROG}.5"
+	rm -f "${DESTDIR}${BASHCOMPDIR}/${PROG}"
+	rm -f "${DESTDIR}${FISHCOMPDIR}/${PROG}.fish"