Revert "package/janet: add -std=c99 to CFLAGS"

This reverts commit b5e8f1c1475b46c8d9b7159aafe983e72d329d29. Indeed,
this commit does not fix the build failure as c_std=c99 is already set
in default_options in meson.build.

The issue is that this parameter is not used for native executables
since meson 0.51.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/janet/janet.mk

@@ -8,8 +8,8 @@ JANET_VERSION = 1.15.0
 JANET_SITE = $(call github,janet-lang,janet,v$(JANET_VERSION))
 JANET_LICENSE = MIT
 JANET_LICENSE_FILES = LICENSE
+
 JANET_INSTALL_STAGING = YES
-JANET_CFLAGS = $(TARGET_CFLAGS) -std=c99
 
 ifeq ($(BR2_STATIC_LIBS),y)
 JANET_CONF_OPTS += -Ddynamic_modules=false