Let's find the patch files whose filenames are not referenced anywhere,
except possibly in 'gnu/local.mk':

$ for patch in gnu/packages/patches/*; do result=$(grep -rI ${patch##*/}) \
&& test ${result%%:*} != "gnu/local.mk" || echo $patch; done
gnu/packages/patches/gcc-libiberty-printf-decl.patch
gnu/packages/patches/grub-CVE-2015-8370.patch
gnu/packages/patches/grub-freetype.patch
gnu/packages/patches/grub-gets-undeclared.patch
gnu/packages/patches/guile-arm-fixes.patch
gnu/packages/patches/hypre-doc-tables.patch
gnu/packages/patches/hypre-ldflags.patch
gnu/packages/patches/mplayer2-theora-fix.patch
gnu/packages/patches/readline-7.0-mingw.patch
gnu/packages/patches/soprano-find-clucene.patch
gnu/packages/patches/util-linux-CVE-2017-2616.patch
gnu/packages/patches/xf86-video-intel-compat-api.patch
gnu/packages/patches/xf86-video-intel-glibc-2.20.patch

Specific notes for each file:

gcc-libiberty-printf-decl.patch: Added in
b810a85019ab3c4ee1f889d0751b8eb06157dadc and mentioned in the
commit message but never used, AFAICT. Ludo?

gnu/packages/patches/grub-CVE-2015-8370.patch,
gnu/packages/patches/grub-freetype.patch,
gnu/packages/patches/grub-gets-undeclared.patch: Unused since grub was
updated to 2.02~beta3 in 4c7260011e219eb3ebf5da976ab107dbd5997829. 

gnu/packages/patches/guile-arm-fixes.patch: Unused since guile was
updated to 2.0.12 in 923d9caa1800fb48b1745cf9ef0fd294a423d4cd,
4f07bb6b433a7fa2fdcb5f465f7b9dfdf0140dae.

gnu/packages/patches/hypre-doc-tables.patch,
gnu/packages/patches/hypre-ldflags.patch: Added but not used in
528bb46436be39a4f4d3e7b3e908c34e92cca5f8. I pinged Eric on #guix.

gnu/packages/patches/mplayer2-theora-fix.patch: Unused since
a6d605e648643cd98582bf1e339bf60706d7c6c6

gnu/packages/patches/readline-7.0-mingw.patch: Introduced in
ae12d586275cdd96db23fb01bf840b2055b5b979, but never used. Jan, can you
take a look?

gnu/packages/patches/soprano-find-clucene.patch: Unused since
dbbb45dd7318528ea92a56992be789d0b5cf2120.

gnu/packages/patches/util-linux-CVE-2017-2616.patch: Unused since
util-linux was updated to 2.29.2 in
83090f6ae7eb5353049b253f74670d72d9387463. The history of this update is
very confusing.

gnu/packages/patches/xf86-video-intel-compat-api.patch,
gnu/packages/patches/xf86-video-intel-glibc-2.20.patch: Unused since
6cc328b43b1ae2cc1d201844cff7c619187cca5e.