feat: 不再对weui-btn_plain进行支持，可使用weui-btn代替

src/style/base/variable/weui-button.less

@@ -59,9 +59,3 @@
 @weuiBtnWarnBg: @weuiBtnDefaultBg;
 @weuiBtnWarnActiveBg: @weuiBtnDefaultActiveBg;
 @weuiBtnwarnDisabledBg: @weuiBtnDefaultDisabledBg;
-
-// plain-btn
-@weuiBtnPlainPrimaryColor: var(--weui-BRAND);
-@weuiBtnPlainPrimaryBorderColor: var(--weui-BRAND);
-@weuiBtnPlainDefaultColor: var(--weui-BTN-DEFAULT-BG);
-@weuiBtnPlainDefaultBorderColor: var(--weui-BTN-DEFAULT-BG);

src/style/widget/weui-button/weui-btn_plain.less

@@ -1,43 +0,0 @@
-/*
-* Tencent is pleased to support the open source community by making WeUI available.
-* 
-* Copyright (C) 2017 THL A29 Limited, a Tencent company. All rights reserved.
-* 
-* Licensed under the MIT License (the "License"); you may not use this file except in compliance
-* with the License. You may obtain a copy of the License at
-* 
-*       http://opensource.org/licenses/MIT
-* 
-* Unless required by applicable law or agreed to in writing, software distributed under the License is
-* distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
-* either express or implied. See the License for the specific language governing permissions and
-* limitations under the License.
-*/
-
-@import "../../base/fn";
-
-.weui-btn_plain-primary {
-    color: @weuiBtnPlainPrimaryColor;
-    border: 1px solid @weuiBtnPlainPrimaryBorderColor;
-    &:not(.weui-btn_plain-disabled):active {
-		background-color:rgba(0,0,0,.1);
-    }
-    &:after {
-        border-width: 0;
-    }
-}
-
-.weui-btn_plain-default {
-    color: @weuiBtnPlainDefaultColor;
-    border: 1px solid @weuiBtnPlainDefaultBorderColor;
-    &:not(.weui-btn_plain-disabled):active {
-		background-color:rgba(0,0,0,.1);
-    }
-    &:after {
-        border-width: 0;
-    }
-}
-.weui-btn_plain-disabled{
-    color:rgba(0,0,0,.2);
-    border-color:rgba(0,0,0,.2);
-}

src/style/widget/weui-button/weui-button.less

@@ -21,7 +21,6 @@
 @import "weui-btn_warn";
 @import "weui-btn_disabled";
 @import "weui-btn_loading";
-@import "weui-btn_plain";
 @import "weui-btn_cell";
 
 button, input {
@@ -36,10 +35,6 @@ button, input {
     &.weui-btn_inline,&.weui-btn_mini {
         width: auto;
     }
-    &.weui-btn_plain-primary,&.weui-btn_plain-default{
-        border-width: 1px;
-        background-color: transparent;
-    }
 }
 
 .weui-btn_mini {