Remove unanimous MAM display

For some time users could choose to have the old way "unanimous" where
all the MUC history is just grey (or whatever was set). Now it is always
just displayed like regular new incoming MUC text.

src/config/preferences.c

@@ -1755,7 +1755,6 @@ _get_group(preference_t pref)
         case PREF_STATUSBAR_ROOM:
         case PREF_TITLEBAR_MUC_TITLE_JID:
         case PREF_TITLEBAR_MUC_TITLE_NAME:
-        case PREF_HISTORY_COLOR_MUC:
         case PREF_SLASH_GUARD:
             return PREF_GROUP_UI;
         case PREF_STATES:
@@ -2065,8 +2064,6 @@ _get_key(preference_t pref)
             return "sendfile";
         case PREF_CORRECTION_ALLOW:
             return "correction.allow";
-        case PREF_HISTORY_COLOR_MUC:
-            return "history.muc.color";
         case PREF_AVATAR_CMD:
             return "avatar.cmd";
         case PREF_SLASH_GUARD:
@@ -2207,8 +2204,6 @@ _get_default_string(preference_t pref)
             return "automatic";
         case PREF_COLOR_NICK:
             return "false";
-        case PREF_HISTORY_COLOR_MUC:
-            return "unanimous";
         case PREF_AVATAR_CMD:
             return "xdg-open";
         default:

src/config/preferences.h

@@ -167,7 +167,6 @@ typedef enum {
     PREF_OMEMO_SENDFILE,
     PREF_OCCUPANTS_WRAP,
     PREF_CORRECTION_ALLOW,
-    PREF_HISTORY_COLOR_MUC,
     PREF_AVATAR_CMD,
     PREF_SLASH_GUARD,
     PREF_MAM,