Postpone introducing common StringBuilder.capacity() function

Its support complicates JS StringBuilder implementation with no actual
performance improvements. Benefits of having capacity() function
in common code are also not completely clear.

Relates to KT-33069
#KT-40168

libraries/stdlib/js/src/kotlin/text/StringBuilderJs.kt

@@ -17,7 +17,6 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      * In Kotlin/JS implementation of StringBuilder the initial capacity has no effect on the further performance of operations.
      */
     actual constructor(capacity: Int) : this() {
-        this.asDynamic()._capacity = capacity
     }
 
     /** Constructs a string builder that contains the same characters as the specified [content] char sequence. */
@@ -135,9 +134,10 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * In Kotlin/JS implementation of StringBuilder the value returned from this method may not indicate the actual size of the backing storage.
      */
-    @SinceKotlin("1.4")
-    @WasExperimental(ExperimentalStdlibApi::class)
-    actual fun capacity(): Int = if (this.asDynamic()._capacity !== undefined) maxOf(this.asDynamic()._capacity, length) else length
+    @SinceKotlin("1.3")
+//    @ExperimentalStdlibApi
+    @Deprecated("Obtaining StringBuilder capacity is not supported in JS and common code.", level = DeprecationLevel.ERROR)
+    actual fun capacity(): Int = length
 
     /**
      * Ensures that the capacity of this string builder is at least equal to the specified [minimumCapacity].
@@ -151,9 +151,6 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
     @SinceKotlin("1.4")
     @WasExperimental(ExperimentalStdlibApi::class)
     actual fun ensureCapacity(minimumCapacity: Int) {
-        if (minimumCapacity > capacity()) {
-            this.asDynamic()._capacity = minimumCapacity
-        }
     }
 
     /**
@@ -367,9 +364,6 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
     @SinceKotlin("1.4")
     @WasExperimental(ExperimentalStdlibApi::class)
     actual fun trimToSize() {
-        if (this.asDynamic()._capacity !== undefined) {
-            this.asDynamic()._capacity = length
-        }
     }
 
     override fun toString(): String = string