build: make -Wsign-conversion an error (OpenSSF)

Before this patch, cmake/autotools builds made an exception for this
warning to not cause an error.

The codebase is warning-free now, so this patch deletes this exception.

Follow-up [...]
Follow-up to3829759bd042c03225ae862062560f568ba1a231 curl#12489
Closes #xxxxx

CMake/PickyWarnings.cmake

@@ -98,7 +98,6 @@ if(PICKY_COMPILER)
       -Wold-style-definition               # clang  2.7  gcc  3.4
       -Wredundant-decls                    # clang  2.7  gcc  4.1
       -Wsign-conversion                    # clang  2.9  gcc  4.3
-        -Wno-error=sign-conversion                                              # FIXME
       -Wstrict-prototypes                  # clang  1.0  gcc  3.3
     # -Wswitch-enum                        # clang  2.7  gcc  4.1               # Not used because this basically disallows default case
       -Wtype-limits                        # clang  2.7  gcc  4.3

m4/curl-compilers.m4

@@ -831,7 +831,6 @@ AC_DEFUN([CURL_SET_COMPILER_WARNING_OPTS], [
           dnl Only clang 2.9 or later
           if test "$compiler_num" -ge "209"; then
             CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [sign-conversion])
-            tmp_CFLAGS="$tmp_CFLAGS -Wno-error=sign-conversion"          # FIXME
             CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [shift-sign-overflow])
           # CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [padded])  # Not used because we cannot change public structs
           fi
@@ -1023,7 +1022,6 @@ AC_DEFUN([CURL_SET_COMPILER_WARNING_OPTS], [
             CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [clobbered ignored-qualifiers])
             CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [conversion trampolines])
             CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [sign-conversion])
-            tmp_CFLAGS="$tmp_CFLAGS -Wno-error=sign-conversion"          # FIXME
             CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [vla])
             dnl required for -Warray-bounds, included in -Wall
             tmp_CFLAGS="$tmp_CFLAGS -ftree-vrp"