style(block-kit): omit emoji arg where the docs example omits emoji

Drop the explicit emoji=None on plain_text objects whose docs.slack.dev
reference example omits the emoji field (feedback buttons, icon button,
rich text input placeholder). slack_sdk drops None fields, so the
serialized payload is unchanged — this only removes redundant source.
emoji=True is kept where the docs example shows "emoji": true.

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>

Author: zimeg

block-kit/src/elements/feedback_buttons.py

@@ -18,12 +18,12 @@ def example01() -> ContextActionsBlock:
             FeedbackButtonsElement(
                 action_id="feedback_buttons_1",
                 positive_button=FeedbackButtonObject(
-                    text=PlainTextObject(text="Good", emoji=None),
+                    text=PlainTextObject(text="Good"),
                     value="positive_feedback",
                     accessibility_label="Mark this response as good",
                 ),
                 negative_button=FeedbackButtonObject(
-                    text=PlainTextObject(text="Bad", emoji=None),
+                    text=PlainTextObject(text="Bad"),
                     value="negative_feedback",
                     accessibility_label="Mark this response as bad",
                 ),

block-kit/src/elements/icon_button.py

@@ -14,7 +14,7 @@ def example01() -> ContextActionsBlock:
         elements=[
             IconButtonElement(
                 icon="trash",
-                text=PlainTextObject(text="Delete", emoji=None),
+                text=PlainTextObject(text="Delete"),
                 action_id="delete_button",
                 value="delete_item",
             ),

block-kit/src/elements/rich_text_input.py

@@ -21,7 +21,7 @@ def example01() -> View:
                         "trigger_actions_on": ["on_character_entered"],
                     },
                     focus_on_load=True,
-                    placeholder=PlainTextObject(text="Enter text", emoji=None),
+                    placeholder=PlainTextObject(text="Enter text"),
                 ),
                 label=PlainTextObject(text="Label", emoji=True),
             ),