package/boost: drop no longer needed patch 0004-workaround-musl-bug.patch

package/boost: drop no longer needed patch 0004-workaround-musl-bug.patch

The 0004-workaround-musl-bug.patch was added because musl's <sched.h>
was using memset(), but without including <string.h>. So including
just <sched.h> was causing a build failure.

However, this issue in musl has been fixed in upstream musl commit
48be5b6313d7b827acf555769e93b389fa9f6307 ("fix use of memset without
declaration in sched.h cpu set macros"). This commit was part of musl
1.1.17, which has been released a while ago. Both 2019.02.x, 2019.05.x
and master use a newer musl version.

So the workaround in Boost can be dropped.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>