MDEV-16294: remove INSTALL SONAME IF EXISTS option

As it isn't used. This leaves a less clustered syntax
for a INSTALL IF EXISTS SONAME option which could be added later.
This option could be checking the existance of the soname rather than the
installed plugin name which the rest of the INSTALL IF NOT EXISTS
is focused around.`

mysql-test/main/plugin.result

@@ -361,12 +361,11 @@ Level	Code	Message
 Note	1305	PLUGIN example does not exist
 UNINSTALL PLUGIN example;
 ERROR 42000: PLUGIN example does not exist
-INSTALL SONAME IF NOT EXISTS 'ha_example';
+INSTALL SONAME 'ha_example';
 select PLUGIN_NAME,PLUGIN_STATUS,PLUGIN_TYPE from information_schema.plugins where plugin_library like 'ha_example%';
 PLUGIN_NAME	PLUGIN_STATUS	PLUGIN_TYPE
 EXAMPLE	ACTIVE	STORAGE ENGINE
 UNUSABLE	ACTIVE	DAEMON
-INSTALL SONAME IF NOT EXISTS 'ha_example';
 UNINSTALL SONAME IF EXISTS 'ha_example';
 UNINSTALL SONAME IF EXISTS 'ha_example';
 Warnings:

mysql-test/main/plugin.test

@@ -292,13 +292,9 @@ SHOW WARNINGS;
 --error 1305
 UNINSTALL PLUGIN example;
 
-INSTALL SONAME IF NOT EXISTS 'ha_example';
-
+INSTALL SONAME 'ha_example';
 select PLUGIN_NAME,PLUGIN_STATUS,PLUGIN_TYPE from information_schema.plugins where plugin_library like 'ha_example%';
 
-# note: installing soname multiple times doesn't cause errors or warnings
-INSTALL SONAME IF NOT EXISTS 'ha_example';
-
 UNINSTALL SONAME IF EXISTS 'ha_example';
 
 UNINSTALL SONAME IF EXISTS 'ha_example';

sql/sql_yacc.yy

@@ -17500,15 +17500,12 @@ install:
             lex->comment= $4;
             lex->ident= $6;
           }
-        | INSTALL_SYM SONAME_SYM opt_if_not_exists TEXT_STRING_sys
+        | INSTALL_SYM SONAME_SYM TEXT_STRING_sys
           {
             LEX *lex= Lex;
-            lex->create_info.init();
-            if (lex->add_create_options_with_check($3))
-              MYSQL_YYABORT;
             lex->sql_command= SQLCOM_INSTALL_PLUGIN;
             lex->comment= null_clex_str;
-            lex->ident= $4;
+            lex->ident= $3;
           }
         ;