remove use of buggy .SECONDARY special target in makefile

this functionality is affected by GNU make bug #30653, "intermediate
files incorrectly pruned in parallel builds". on affected versions of
make, parallel builds attempt to compile source files before
alltypes.h is generated.

as noted with commit a91ebdcf, which
added the use of .SECONDARY, suppression of removal of "intermediate"
files does not seem to be needed at present. if it is needed in the
future, it should be achievable by explicitly mentioning their names
as targets or prerequisites.

Makefile

@@ -203,6 +203,4 @@ musl-git-%.tar.gz: .git
 musl-%.tar.gz: .git
 	 git archive --format=tar.gz --prefix=$(patsubst %.tar.gz,%,$@)/ -o $@ v$(patsubst musl-%.tar.gz,%,$@)
 
-.SECONDARY:
-
 .PHONY: all clean install install-libs install-headers install-tools