Revert "package/libmicrohttpd: fix pthread detection"

This reverts commit 834464792e691a243d456335083b2a8b3363e261, the build
failure is not related to the bump to 0.9.75 but to or1k and gcc 10.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/libmicrohttpd/libmicrohttpd.mk

@@ -12,10 +12,6 @@ LIBMICROHTTPD_INSTALL_STAGING = YES
 LIBMICROHTTPD_CONF_OPTS = --disable-curl --disable-examples
 LIBMICROHTTPD_CFLAGS = $(TARGET_CFLAGS) -std=c99
 
-ifeq ($(BR2_TOOLCHAIN_HAS_THREADS),y)
-LIBMICROHTTPD_CFLAGS += -D_REENTRANT
-endif
-
 LIBMICROHTTPD_CONF_ENV += CFLAGS="$(LIBMICROHTTPD_CFLAGS)"
 
 ifeq ($(BR2_PACKAGE_LIBMICROHTTPD_SSL),y)