Drop mappings declaration for unobfuscated 26.2

Starting with 26.1 Mojang ships Minecraft unobfuscated, and Yarn /
Intermediary are no longer published past 1.21.11. Loom does not need
a mappings dependency in that mode: the Mojang-named source compiles
directly. Remove the mappings line, drop the yarn_mappings property,
revert the client entrypoint to Mojang names (Minecraft, Component,
ChatFormatting, gui.getChat(), getUser().getName()), and rewrite the
README note so it stops claiming Mojang mappings via Loom.

build.gradle

@@ -30,7 +30,6 @@ repositories {
 
 dependencies {
     minecraft "com.mojang:minecraft:${project.minecraft_version}"
-    mappings "net.fabricmc:yarn:${project.yarn_mappings}:v2"
     modImplementation "net.fabricmc:fabric-loader:${project.loader_version}"
     modImplementation "net.fabricmc.fabric-api:fabric-api:${project.fabric_version}"
 }