chore: remove a workaround for a file input with "required" attribute.

HtmlUnit/htmlunit#76 has been fixed.

Follow-up to #1251

Author: php-coder

src/test/robotframework/series/creation/logic-user.robot

@@ -13,9 +13,6 @@ Create series by filling only required fields
 	Select From List By Label  id:category  Sport
 	Input Text                 id:quantity  2
 	Choose File                id:image     ${MAIN_RESOURCE_DIR}${/}test.png
-	# Workaround for an invalid form when it contains required file input:
-	# https://github.com/HtmlUnit/htmlunit/issues/76
-	Remove Element Attribute   image  required
 	Submit Form                id:add-series-form
 	Element Text Should Be     id:category_name  Sport
 	Element Text Should Be     id:quantity       2
@@ -28,9 +25,6 @@ Create series by filling all fields
 	Input Text                 id:quantity  3
 	Unselect Checkbox          id:perforated
 	Choose File                id:image  ${MAIN_RESOURCE_DIR}${/}test.png
-	# Workaround for an invalid form when it contains required file input:
-	# https://github.com/HtmlUnit/htmlunit/issues/76
-	Remove Element Attribute   image  required
 	Click Element              id:specify-issue-date-link
 	Select From List By Value  id:day    8
 	Select From List By Value  id:month  9

src/test/robotframework/series/creation/misc-user.robot

@@ -51,9 +51,6 @@ Catalog numbers should ignore duplicate values
 	Select From List By Label  id:category  Sport
 	Input Text                 id:quantity  2
 	Choose File                id:image  ${MAIN_RESOURCE_DIR}${/}test.png
-	# Workaround for an invalid form when it contains required file input:
-	# https://github.com/HtmlUnit/htmlunit/issues/76
-	Remove Element Attribute   image  required
 	Click Element              id:add-catalog-numbers-link
 	Input Text                 id:michelNumbers    104,105,104
 	Input Text                 id:scottNumbers     114,115,114
@@ -74,9 +71,6 @@ Catalog numbers should accept existing numbers
 	Select From List By Label  id:category  Sport
 	Input Text                 id:quantity  2
 	Choose File                id:image  ${MAIN_RESOURCE_DIR}${/}test.png
-	# Workaround for an invalid form when it contains required file input:
-	# https://github.com/HtmlUnit/htmlunit/issues/76
-	Remove Element Attribute   image  required
 	Click Element              id:add-catalog-numbers-link
 	Input Text                 id:michelNumbers    99
 	Input Text                 id:scottNumbers     99